Ozone depletion in January 2026: impact of energetic particle precipitation

Energetic particle precipitation (EPP) during geomagnetic storms plays a significant role in accelerating ozone depletion in the high-latitude upper atmosphere. During the January 2026 geomagnetic storm with a sudden commencement reached NOAA G4 level following an X1.9-class solar flare on January 18 and a fast coronal mass ejection arrived at Earth. The storm commenced on January 19, producing an AE index up to 2,597 nT and a severe storm levels globally (NOAA Kp 8-). The storm persisted through January 20, producing a | Dst | index minimum of 236 nT, characteristic of an intense event. The Aura Microwave Limb Sounder (MLS) instrument provides simultaneous measurements of chemical species from the upper troposphere to the mesosphere, enabling detailed investigation of energetic particle precipitation effects on atmospheric composition. Using data from MLS/Aura collected in January 2026, we have analyzed the HO 2 , HNO 3 , and ozone variability over the high-latitude bands between 60 o and 80 o in both the Northern and Southern Hemispheres. MLS/Aura observations show EPP influence in both hemispheres, with it being more pronounced in the Northern Hemisphere. Instantaneous EPP-induced ozone depletion up to 70% is observed in the altitudinal range from 0.01 hPa to 0.1 hPa, and after this EPP ozone did not recover until the end of the month.

The energetic particle precipitation (EPP) induced by coronal mass ejection (CME) and geomagnetic storms plays a significant role in the upper atmosphere including ionization, excitation, dissociation, and recombination processes, as well as subsequent chemical reactions.

The ability of energetic particle to sporadically enhance ionization of the Earth’s polar atmosphere lead to the formation of odd forms of hydrogen (HO x = H + OH + HO 2) and odd forms of nitrogen (NO x = N + NO + NO 2) (Porter et al., 1976; Solomon et al., 1981; Crutzen et al., 1975; Aikin, 1994). Both, HO x and NO x play a fundamental role in the ozone balance by driving efficient catalytic cycles of ozone depletion in the upper atmosphere at high latitudes in both hemispheres (e.g.,Randall et al., 2005; Rozanov et al., 2012; Mironova et al., 2015; Matthes et al., 2017). The lifetime of HO x is relatively short, therefore their enhancement is highly localized in time and space, and can be observed only during EPP events in the region where the ionization occurs. The OH radical is the principal atmospheric oxidant, affecting both aerosols and ozone in complex ways. Moreover, under nighttime conditions the low background in HO x allows particles-induced changes to be clearly identified. The main processes responsible for the HO x formation were described by Porter et al. (1976); Solomon et al. (1981); Aikin (1994), and more recently by Sinnhuber et al. (2012). Both, HO x and NO x play a fundamental role in the ozone balance by driving efficient catalytic cycles of O 3 destruction. NO x is more stable in the absence of sunlight and can be transported by atmospheric winds during polar nights, therefore NO x enhancement can be observed also after EPP events and far away from the production area. The ozone response to HO x and NO x enhancements depends on the altitude as well as on the horizontal and vertical air mass redistribution within the polar vortex. The main catalytic reactions with NO x lead to the destruction of ozone:

Reaction 1: NO + O 3 → NO 2 + O 2

Reaction 2: NO 2 + O → NO + O 2

Overall reaction: O 3 + O → 2 O 2.

The catalytic reactions with odd hydrogen HO x are also effective catalyst for the destruction of O 3, especially in the mesosphere:

Reaction 1: OH + O 3 → HO 2 + O 2

Reaction 2: HO 2 + O → OH + O 2

Overall reaction: O 3 + O → 2 O 2.

Nitric acid (HNO 3) formation in polar night regions during the EPP is controlled by the ion chemistry, and the effects can cover the entire polar region (Solomon et al., 1981; Orsolini et al., 2009; Verronen et al., 2011; von Clarmann et al., 2013; Doronin et al., 2025). HNO 3 serves as a key reservoir species for the reactive nitrogen family (NO y = N, NO, NO 2, NO 3, HNO 3, HNO 4, ClNO 3, N 2 O 5) (Porter et al., 1976). In the Earth’s atmosphere. Its importance is particularly pronounced in the middle atmosphere, where it plays a central role in several critical physicochemical processes. Possible mechanisms that could produce HNO 3 during and after EPP include reactions with positive ion clusters and recombination between positive and negative ions (Solomon et al., 1981). Simultaneously, the ionization also produces free electrons that readily attach to electronegative species (especially O 2 and NO 2), forming negative ions such as O 2 − and NO 3 −. These negative ions undergo further clustering and reactions with water and other acids to form larger ion complexes, including NO 3 − (HNO 3) m (H 2 O) n. The mutual neutralization (recombination) of these massive positive and negative ion clusters is a highly efficient process that releases a substantial amount of internal energy, causing the clusters to fragment and evaporate. This fragmentation often results in the ejection of stable neutral molecules, with HNO 3 being one of the most prominent products. This ion-ion recombination pathway is considered particularly effective at high altitudes where the ionic charge density is elevated during EPP events. Notably, HNO 3 is a primary component in the formation of polar stratospheric clouds (PSCs), for example, induced by EPP during the cold winter months (Mironova et al., 2012; Mironova and Usoskin, 2013). These clouds provide surfaces for heterogeneous chemical reactions that activate chlorine species, ultimately leading to catalytic ozone destruction. Furthermore, HNO 3 contributes to denitrification—the permanent removal of nitrogen oxides from the stratosphere through gravitational settling of PSC particles—which prolongs ozone depletion well into spring. Also energetic particle impact leads to the ionization of major atmospheric constituents (N 2 and O 2), generating primary positive ions such as N 2 + and O 2 +. These rapidly undergo a series of ion-molecule reactions with trace neutral species (e.g., water vapor, ozone, and nitrogen oxides), forming complex hydrated proton clusters, such as H + (H 2 O) n and NO + (H 2 O) n. Together, these ion-chemical routes provide a fast, non-photolytic source of HNO 3 that can significantly perturb the nitrogen budget of the polar middle atmosphere on timescales ranging from hours to weeks, thereby influencing the overall chemical balance and ozone recovery dynamics in regions affected by space weather.

Geomagnetic disturbances are one of the key space weather phenomena, consisting of the precipitation of energetic particles toward the polar regions and their collision with atmospheric gases. Numerous studies have shown that enhanced ionization caused by EPP alters the chemical composition of the middle atmosphere and leads to ozone variability (e.g., Funke et al., 2011; Rozanov et al., 2012; Mironova et al., 2015; Sinnhuber et al., 2018; Andersson et al., 2018; Mironova et al., 2021; Grankin et al., 2023; Mironova et al., 2023; Doronin et al., 2024; Doronin et al., 2025; Mironov et al., 2026). While long-term climatological averages of energetic particle precipitation provide valuable baseline information on its cumulative influence on the middle atmosphere, the investigation of individual, distinct EPP events is essential for fundamental scientific and practical reasons. Not all EPP events are created equal. Solar proton events, geomagnetic storm-driven electron precipitation, and relativistic electron dropout events each exhibit unique energy distributions (soft vs. hard spectra), total particle fluxes (ranging from minor enhancements to extreme historical events), duration (from hours to several days). These parameters directly determine the altitude penetration depth of the precipitating particles. Averaging across events obscures these altitude-dependent effects. Investigating separate events allows us to identify these tipping points and understand the time-dependent interplay between ion production and changes of ozone destruction chemial compositions are completely lost in averaged datasets. Thus, a case-study approach remains the gold standard for advancing our understanding of EPP-driven atmospheric change.

The January 2026 geomagnetic storm serves as an exemplary case study for the investigation of separate EPP events. This storm was a significant and multifaceted space weather phenomenon, distinguished not by being the most extreme geomagnetic storm, but by its unique and record-setting combination of characteristics. Therefore in this work, we focus on study the MLS/Aura behavior of the chemical composition of the atmosphere, the HO 2, HNO 3, and ozone variability over the high-latitude bands between 60 o and 80 o in both the Northern (NH) and Southern (SN) Hemispheres during January 2026. These observations will provide a first report of the effect of energetic particle precipitation (EPP) induced by a geomagnetic storm in January 2026 that is classified as a storm with a sudden commencement (SSC).

Geomagnetic storm in January 2026

One of the most significant space weather events of Solar Cycle 25 occurred in January 2026, when a major X1.9 solar flare and subsequent coronal mass ejection (CME) triggered a sudden storm commencement. Geomagnetic storm with a sudden commencement (SSC) reached NOAA G4 level and recorded in ground-based magnetometers. Figure 1 demonstrates temporal variations of the geomagnetic disturbances for January 2026.

The intensity of EPP is largely governed by the north–south orientation of the interplanetary magnetic field’s Bz component. When this component is directed southward in near-Earth space, it serves as a primary driver of significant geoeffective events, often manifesting as geomagnetic disturbances. Prolonged intervals of a strongly southward Bz—oriented oppositely to Earth’s magnetic field—are a critical precondition for intense geomagnetic activity. In particular, the most extreme southward deviations in the IMF Bz are directly associated with the onset of major geomagnetic storms. Geomagnetic indices serve as quantitative measures of variations in Earth’s magnetic field, which arise from solar wind interactions with the magnetosphere, internal magnetospheric dynamics, and magnetosphere–ionosphere coupling. Disturbances in the geomagnetic field are typically characterized using a range of indices—such as AE, Kp, and Dst—each of which is tailored to specific latitudes. These distinct indices reflect different current systems and are associated with particle precipitation of varying origins and energy spectra. Geomagnetic disturbances are characterized by several complementary indices, each capturing different aspects of the Earth’s magnetic response. The AE index monitors polar auroral-zone activity driven by enhanced eastward and westward electrojet currents along the auroral oval. The Dst index, based on low-latitude measurements, tracks field depressions caused by ring current intensification during geomagnetic storms. Finally, the Kp index provides a planetary, hourly measure of global disturbance, derived from the averaged horizontal-component fluctuations at observatories in the subauroral zone, typically located between 50 o and 60 o geomagnetic latitude in both northern and southern hemispheres.

On January 19 the north-south interplanetary magnetic field (IMF) component Bz turned strongly southward (see upper panel of Figure 1) and then turned northward to about 60 nT at the shock this southward phase favored efficient coupling with Earth magnetosphere and explains the rapid escalation of geomagnetic activity. The storm commenced on January 19, producing an AE index up to 2,597 nT and a severe storm level globally (NOAA Kp 8-). The storm persisted through January 20, producing a | Dst | index minimum of 236 nT, characteristic of an intense event. The disturbances drove an intense energetic particle precipitation (EPP) shower that hit Earth on January 18–20, directly depositing high-energy solar protons and magnetospheric electrons into Earth’s upper and middle atmosphere.

CME on 18 January 2026 accompanied solar energetic particle (SEP) event persists January 18-19, 2026 (NOAA data). On January 18 at 2255 UTC the greater than 10 MeV GOES proton flux has remained above the 10 pfu threshold and peaked near January 19 at 1915 UTC with 37,000 pfu, while the greater than 100 MeV channel shows only a modest enhancement and the greater than 500 MeV channel remains near background. The January 2026 solar proton event ranks third in recorded history for > 10 MeV proton flux, behind only the March 1991 (43,000 pfu) and October 1989 (40,000 pfu) events. The SEP event was accompanied by a geomagnetic storm leading to radiation belts magnetospheric electrons precipitation during this period.

The January 2026 storm is described as a case study event with a single, extremely fast Earth-directed CME, producing strong coupling even with predominantly northward IMF during the main phase.

MLS/Aura observations during January 2026

The Earth Observing System (EOS) Microwave Limb Sounder (MLS) is an instrument on NASA’s Aura spacecraft, launched in July 2004 into a quasi-polar sun-synchronous orbit at an altitude of 705 km with an inclination of 98.2 o. MLS measures millimeter and sub-millimeter wavelength thermal emissions from the Earth’s limb, vertically scanning the atmosphere in the orbital plane to provide daily near-global coverage from approximately 80 o NH to 80 o SH. With 15 orbits per day, the instrument performs measurements during both day and night, allowing continuous monitoring of atmospheric composition. The measurement technique is based on limb sounding: MLS looks tangentially through the atmospheric limb, measuring thermal emission at selected frequencies in the millimeter and sub-millimeter spectral ranges (118 GHz–2.5 THz). This approach provides high vertical resolution (about 3–6 km) from the upper troposphere to the mesosphere. Unlike nadir-viewing instruments, limb sounding offers superior sensitivity to trace gases in the upper atmosphere due to the long atmospheric path length and reduced surface interference.

Among the most scientifically valuable MLS/Aura retrieval products are:

HNO 3 (nitric acid): A key reservoir for odd nitrogen (NO y), produced by ion chemistry following energetic particle precipitation. HNO 3 provides a robust tracer for the downward transport of NO y from the mesosphere to the stratosphere, where it participates in catalytic ozone destruction cycles.

HO 2 (hydroperoxyl radical): A critical component of the odd hydrogen (HO x) family, which catalytically destroys ozone in the mesosphere and upper stratosphere. HO 2 enhancements are directly linked to the production of HO x through ion chemistry triggered by energetic particle precipitation.

O 3 (ozone): The primary target species, measured simultaneously with its chemical precursors. Ozone profiles from MLS allow quantification of short-term depletion events and assessment of recovery times following geomagnetic storms. The quality characteristics of the MLS ozone retrievals used in this study were taken from the official NASA MLS data quality document.

In our study, we consider the latest (version 5) MLS/Aura data with increased vertical range for ozone and some other species (Livesey N. et al., 2020). A new version of data retrieval has better quality in the upper mesosphere. Preliminary research has shown that the improvements are sufficient to increase the vertical range recommended for scientific use, see for details Livesey N. J. et al. (2020). For the hydroperoxyl radical, the recommended vertical range is 22–0.046 hPa, which is appropriately used in our study. Changes in hydroperoxyl radical concentration during EPP in the vertical range above 0.046 hPa demonstrate a continuation of the EPP-induced HO 2 increase observed in the recommended vertical range. HNO 3 product in the upper troposphere/lower stratosphere is a noisy by clouds (Livesey N. et al., 2020). The HNO 3 data at 1.0 hPa and lower pressures may be useful under certain enhanced conditions, however, as discussed by Verronen et al. (2011), the signal-to-noise ratio may be improved during large EPP events, making it possible that an increase in HNO 3 is significantly detected. For the ozone pressure range considered here (0.01–0.1 hPa), the vertical resolution is approximately 4–5.5 km. The estimated single-profile precision is 0.4–1.1 ppmv, while the reported accuracy ranges from approximately 100% at 0.01 hPa to 10% at 0.1 hPa, improving toward higher pressures. The MLS quality documentation recommends averaging at the highest altitude levels because of the relatively large uncertainty of individual profiles. Consistent with this recommendation, averaged MLS ozone fields were used, which reduced random errors before further analysis.

The ability of MLS to simultaneously measure multiple chemical species makes it particularly well-suited for investigating the complex photochemical and dynamical responses of the middle atmosphere to energetic particle precipitation events, including those during the January 2026 geomagnetic storm.

During the January 2026 geomagnetic storm with a sudden commencement (SSC) provided an ideal natural laboratory for examining EPP under extreme forcing. In this section, we present the first MLS/Aura observations of the chemical composition of atmospheric ozone and ozone depletion in response to this complex space weather event.

Figure 2 demonstrates the altitudinal and temporal variability of HNO 3 over 60 o - 80 o in both the Northern (NH) and Southern (SH) Hemispheres in January 2026.

Starting from January 12, noticeable changes in HNO 3 concentrations appeared in both hemispheres, whereas before this date the mean values remained close to the background level (< 0.1 ppb) throughout the entire altitude range. From 12 to 15 January, moderate HNO 3 enhancements were observed in both hemispheres. In the NH, localized increases reached approximately 0.3–0.7 ppb, mainly at pressures between about 0.3 and 0.7 hPa. In the SH, similar enhancements were detected over a broader altitude range, with peak values of about 0.5–0.6 ppb near 0.05 hPa and 0.3–0.5 hPa. Between 16 and 17 January, HNO 3 concentrations remained elevated. In the NH, values of about 0.5–0.8 ppb were observed around 0.1 hPa and near 0.5 hPa. In the SH, enhancements of approximately 0.3–0.5 ppb extended from 0.05 to 0.15 hPa, while lower altitudes remained close to 0.3 ppb. The most pronounced changes occurred after the EPP event during January 18–20. In the NH, a strong and rapid increase in HNO 3 was observed on January 18, particularly at pressures between 0.3 and 0.5 hPa, where concentrations rose to about 1.3–1.6 ppb. Elevated values also appeared in the upper mesosphere (around 0.05 hPa), reaching approximately 0.8–1.0 ppb. The enhancement persisted and intensified on January 19–20, with maximum HNO 3 values exceeding 1.5 ppb near 0.4–0.5 hPa. Thus, compared with the pre-EPP period, HNO 3 increased by more than an order of magnitude. A similar response was observed in the SH, although with lower absolute values. Following the EPP event, HNO 3 concentrations increased markedly on 18 January in the lower part of the considered altitude range (0.3–0.7 hPa), reaching about 0.8–0.9 ppb. Enhanced concentrations persisted on January 19–20, with maxima of approximately 0.7–0.9 ppb near 0.3–0.6 hPa and secondary maxima of about 0.6 ppb around 0.04 hPa. After January 20, HNO 3 values in both hemispheres rapidly returned to background levels (< 0.1 ppb), indicating that the observed enhancements were short-lived and associated with the EPP event.

Figure 3 demonstrates the altitudinal and temporal variability of HO 2 over 60 o - 80 o in both the Northern and Southern Hemispheres in January 2026.

Throughout January, HO 2 exhibited substantial day-to-day variability in both hemispheres. Therefore, only the changes associated with the EPP event on January 18 are discussed here. Following the EPP event, a pronounced enhancement of HO 2 was observed in the NH. The increase became evident on January 19 and was most prominent between approximately 0.01 and 0.10 hPa, with the maximum centred near 0.05 hPa. Mean HO 2 values increased from typical background levels of about 0.3–0.5 ppb to 1.1–1.2 ppb, corresponding to an enhancement of approximately 0.7–0.9 ppb. Elevated concentrations were also detected down to about 0.2 hPa, although the magnitude of the increase decreased with increasing pressure. Because wintertime HO 2 concentrations in the Northern Hemisphere are generally very low, the EPP-induced enhancement stands out clearly against the background variability. A response was also observed in the SH after January 18. However, unlike the NH, the Southern Hemisphere was characterized by substantially higher background HO 2 concentrations throughout the month, with typical values of 1.8–2.3 ppb between approximately 0.02 and 0.10 hPa. Following the EPP event, HO 2 increased to approximately 2.5–2.7 ppb, corresponding to an enhancement of about 0.3–0.5 ppb. Instead of producing a distinct localized maximum, the increase appeared as a broader enhancement extending over a relatively wide altitude range between 0.02 and 0.10 hPa. Consequently, the EPP signature is much less obvious in the Southern Hemisphere. Without prior knowledge of the EPP event, this enhancement could easily be interpreted as part of the natural background variability.

Figure 4 demonstrates the altitudinal and temporal variability of O 3 over 60 o - 80 o in both the Northern and Southern Hemispheres in January 2026.

Following the EPP event started on January 18, a pronounced ozone depletion is observed in the NH, with ozone value decreasing to approximately 70% of its pre-event mean value (relative to the average for January1–18). Decline in the NH reaches its peak on January 20. In contrast, the SH exhibits a considerably weaker response, with a maximum ozone reduction of about 20% of the average value for 01.01–18.01, peaking on 19 January. The ozone loss extends across the entire altitude range (0.1–0.01 hPa) shown in Figure 4. This range corresponds to the altitudes where a simultaneous increase in HO 2 content is observed Figure 3. This altitude coincidence strongly suggests that the observed ozone destruction is driven by catalytic hydrogen oxide (HO x) chemistry, initiated by the influx of energetic particles into the polar upper atmosphere. The recovery time for the ozone layer (0.1–0.01 hPa) after its depletion has been extended until the end of the month.

The January 2026 geomagnetic storm offers a unique opportunity to examine instantaneous EPP-driven chemistry on the polar ozone budget. The event was not merely a strong geomagnetic storm but a historically significant and unusually complex space weather event, offering a rare opportunity to study extreme conditions and test our understanding of the Sun-Earth connection.

The goal of this work was to analyze the MLS/Aura observations of multiple chemical species that are well-suited for investigating the complex photochemical and dynamical responses of the middle atmosphere to energetic particle precipitation events, including those during the January 2026 geomagnetic storm. Our analysis of the January 2026 complex space weather event shows that EPP-induced HNO 3 and HO 2 increases lead to instantaneous ozone destruction in the mesosphere. Noticeable changes in the volume mixing ratio of HNO 3 in the stratosphere, reaching approximately 1 ppb, were observed in both hemispheres, beginning on January 12 and reaching a maximum on January 20. The onset of these changes can be explained by geomagnetic disturbances beginning on January 10, see variations in the Dst index in Figure 1, which showed a minimum value on January 12. These disturbances are confirmed by the response of the entire current system according to the Kp index. HNO 3 increases significantly during solar proton event, January 18 -20, and it appears that there are two layers where HNO 3 increases with one around 0.0464 hPa and the other around 0.464 hPa. However based on the quality of the data report it can be an artifact in the HNO 3 estimation. HO 2 increases observed in the altitudinal range from 0.01 hPa to 0.1 hPa (approximately 65–85 km) after EEP. MLS/Aura observations show EPP influence in both hemispheres, with it being more pronounced in the Northern Hemisphere with ozone did not recover until the end of the month. The prolonged recovery time suggests that the EPP event not only caused immediate photochemical ozone loss through HO x catalytic cycles but may have also perturbed the dynamical circulation, influencing the transport of ozone and its precursors at high latitudes.

Figure 4 is central to demonstrating the direct effect of the January 2026 energetic particle precipitation event on mesospheric ozone. Our results reveal more substantial ozone depletion in the Northern Hemisphere. This asymmetry is consistent with the dependence of dissociation processes on both UV photodissociation and energetic particle precipitation (Brasseur and Solomon, 2005). In the Southern Hemisphere, the concomitant UV radiation renders the EPP effects less discernible. Energetic particle deposition from solar proton events and geomagnetic storms alike enhances HNO 3 and HO x, while also driving NO y production in the thermosphere and upper mesosphere. The resulting reactive nitrogen species are transported downward, first to the middle mesosphere and eventually to the stratosphere, where they contribute additional ozone loss (Brasseur and Solomon, 2005). Hence, mesospheric ozone depletion is driven by both HO x and NO y catalytic cycles.

Previous studies of ozone depletion caused by individual EPP events based on MLS/Aura observations of other January years (e.g., the January 2005 and 2012 EPP events) (Doronin et al., 2025) observed similar signs of depletion, despite the fact that the January 18-19, 2026 EPP ranks third in recorded history for proton flux > 10 MeV (37,000 pfu), surpassed only by the events of March 1991 (43,000 pfu) and October 1989 (40,000 pfu).

The maximum depletion of Northern Hemisphere mesospheric ozone in the 0.1–0.01 hPa altitude range varied significantly across the three events, showing a clear trend toward increasing intensity from 2005 to 2026. During the January 2005 event, MLS/Aura observations recorded an ozone reduction of approximately 60%. During the January 2012 event, the depletion was somewhat weaker, reaching approximately 50% at the same pressure level. The January 2026 event demonstrated the most pronounced impact, with an ozone reduction in this altitude range of approximately 70%. This gradual increase in ozone depletion correlates with the intensity of associated energetic particle deposition, as the January 2026 event was characterized by a record solar proton flux of 37,000 pfu—significantly exceeding the fluxes observed in 2005 and 2012. The recovery time of the mesospheric ozone layer can be prolonged by NO x production associated with electron precipitation during the recovery phase of the geomagnetic storm. In the altitude range of 0.1–0.01 hPa, in the upper mesosphere, HO x induced catalytic ozone destruction is most efficient after energetic particle deposition. The comparison of amounts of HO 2 generated during the three events in January 2005, 2012 and 2026 shows that HO 2 induced by EPP most extended during January 2026. The stronger depletion observed in 2026 suggests that the higher particle flux led to enhanced formation of HO x and NO y species, which in turn led to more efficient ozone destruction via catalytic cycles. These observations demonstrate a direct relationship between the intensity of solar proton events and the magnitude of mesospheric ozone depletion, with the January 2026 event representing the most severe impact of the three studied events.

This comparison supports the conclusion that the January 2026 event is one of the most powerful in the satellite era, with a commensurate impact on mesospheric chemistry.