Impact of High Intensity Long-Duration Continuous Auroral Electrojet Activity (HILDCAAs) on relativistic electrons of the Radiation Belt of Earth during Van Allen probe era

The acceleration of electrons in the outer radiation belt of the Earth remains a complex and intriguing topic for researchers. The existence of this belt of charged particles was first confirmed by the Explorer I mission, leading to its subsequent naming in honor of James Van Allen. The Van Allen outer radiation belt primarily consists of particles originating from the solar wind that become trapped by the planet’s magnetosphere. The Van Allen Probe mission, also known as Radiation Belt Storm Probes (RBSP) (Breneman et al., 2022) launched by NASA aimed to study the behavior of these trapped particles in response to changing solar activity and transient events. The outer radiation belt is highly dynamic and varies minute to solar cycle time scales (Baker et al., 2014; Kozyreva & Kleimenova, 2009; Hajra et al., 2014b). The solar wind transients like coronal mass ejections, Corotating Interaction regions, shocks, etc, (Murphy et al., 2018; Pandya et al., 2019; Schiller et al., 2018; Bhaskar et al., 2021) and geomagnetic activity within magnetosphere like substorm, plasma waves are primary causes of the modulation of the particle fluxes within the radiation belts (Ma et al., 2022; Summers et al., 2004; Fok et al., 2001). Freeman Jr (1964) observed that high-energy (MeV) electron fluxes dropped during the storm’s main phase and increased back to or above pre-storm levels during the storm’s recovery phase over a significant portion of the outer radiation belt (L$>$4). Also, another study by Brautigam & Albert (2000) suggests that radial diffusion (Osmane & Lejosne, 2021; Ozeke et al., 2014, 2012) of relativistic electrons propagates outer boundary variations into the heart of the outer radiation belt during the storm recovery phase, leading to both significant decreases and increases in the $<$1 MeV electron fluxes throughout that region. However, the electron fluxes could drop rapidly by several orders of magnitude and stay low even after the storm is over (Turner et al., 2012).

Even without storms, enhanced whistler-mode chorus waves during prolonged substorm activity can generate relativistic electrons in the Earth’s outer radiation belt in general (Ma et al., 2022; Murphy et al., 2018). Mathie & Mann (2000) reported a rare radiation belt occurrence in which there were no discernible VLF chorus waves but strong ULF waves. An average augmentation of relativistic electron fluxes was observed, which, within the context of the VLF chorus wave-driven local acceleration, might be readily misunderstood. The thorough simulation and the high-resolution data demonstrate that the ULF waves could radially spread and accelerate the radiation belt electrons effectively in the absence of VLF chorus waves. Even Mann et al. (2016)’s study showed how the third radiation belt formed as a direct result of the extraordinarily rapid outward ULF wave transfer during storms. According to Fukizawa et al. (2020), there is a statistically significant correlation, indicating that the dynamics of electron precipitation into the atmosphere are influenced by electron cyclotron harmonic waves. This new viewpoint implies that the dynamics of the ultra-relativistic third radiation belt may not require high-frequency wave-particle scattering loss into the atmosphere (Murphy et al., 2020).

There exists a distinct class of solar wind structures that significantly impacts the Earth’s magnetosphere-ionosphere system over extended periods. These structures are characterized by prolonged intervals of fluctuating southward components of the interplanetary magnetic field (IMF $B_{z}$). When the southward $B_{z}$ component persists, it facilitates the transfer of solar wind energy into the magnetosphere, resulting in sustained auroral activity. This phenomenon is referred to as High-Intensity Long-Duration Continuous Auroral Electrojet Activity (HILDCAA) (Tsurutani & Gonzalez, 1987; Tsurutani et al., 2004; Søraas et al., 2004; Guarnieri, 2006; Hajra et al., 2013, 2014a). These events play a crucial role in geomagnetic activity in the auroral region, occurring without significant development of ring current. HILDCAA events are geomagnetic disturbances characterized by intense auroral activity and thus sustained high levels of the AE/AL index, often exceeding 1000 nT amplitude. These events typically last for a minimum of two days, during which the AE index remains above 200 nT without dropping below this threshold for more than two hours. Unlike geomagnetic storms, HILDCAAs occur outside of the main storm phases and are primarily known for their prolonged and continuous nature.

HILDCAA events are particularly noteworthy for their impact on the Earth’s radiation belts, especially on relativistic electrons within the energy range of approximately 1 to 5 MeV. The continuous substorm activity associated with HILDCAAs generates plasma waves such as ULF (ultra-low-frequency) waves, which play a critical role in the acceleration and transport of these high-energy electrons. The frequent and intense auroral activity during HILDCAAs is a key driver of these ULF waves, leading to significant changes in the dynamics of the radiation belts(Hajra et al., 2015a; Su et al., 2015). As mentioned by Summers et al. (2004) HILDCAA events during which prolonged substorms and enhanced chorus activity occur, are associated with relativistic electron flux enhancements. At geosynchronous orbit, the HILDCAA events are found to be related to an enhancement of relativistic (E $>$ 2 MeV) electron fluxes in the magnetosphere. Recently, Hajra et al. (2024) studied ultra-relativistic electrons of energy $\sim$2-20 MeV. They analyzed electron acceleration for HILDCAA events based on their duration. HILDCAA events of longer periods are associated with larger energy of accelerated relativistic electrons. At the onset of HILDCAA occurred in 2019 February 1-3, relativistic electron fluxes of magnitude $\sim$2.0-2.9 MeV declined from the magnetosphere at around L$>=$4. The study indicated that electron depletion is possibly due to the magnetopause shadowing effect and electron interaction with EMIC waves. According to Milan et al. (2023), every HILDCAA onset is accompanied by substorm-specific changes in open magnetic flux, reconnection rates on both the dayside and nightside, the cross-polar cap potential, and the AL index. Their study highlights the close relationship between substorm activity and AE/AL disturbances during HILDCAAs, suggesting that the magnetic spikes observed during these events are directly associated with jumps in the AE/AL indices and the generation of ULF waves.

Thus, based on past studies, HILDCAA events are strongly correlated with increases in relativistic electron fluxes in the outer radiation belt(Hajra et al., 2015b). However, despite the growing understanding of HILDCAA events, several questions remain open, particularly regarding their general and long-term impact on radiation belt relativistic electrons. How do these events influence the dynamics of relativistic electrons over extended periods? What role do ULF waves play in the acceleration and loss of these particles during HILDCAA events? Addressing these questions is crucial for understanding the broader implications of HILDCAA events on space weather and the Earth’s radiation environment.

The present study attempts to address some of these open questions by investigating the impact of HILDCAA events on relativistic electrons in the Earth’s radiation belt specifically during NASA’s Van Allen Probes era. By analyzing in situ particle measurements for all HILDCAAs occurring in this period, we seek to shed light on the mechanisms through which they influence the behavior of high-energy particles, contributing to a better understanding of the complex interactions between geomagnetic activity induced by HILDCAAs and radiation belt dynamics.

The HILDCAA events have been identified by following the strict criteria given by Tsurutani and Gonzalez (Tsurutani & Gonzalez, 1987). The AE and Sym-H indices have been obtained from https://wdc.kugi.kyoto-u.ac.jp/. The near-earth solar wind conditions during HILDCAA have been studied using the data available at https://omniweb.gsfc.nasa.gov/. The OMNIWeb database is already time-shifted to the bow shock’s nose and the measured quantities in the GSM coordinate system have been used for further analysis (Russell, 1971; Hapgood, 1992).

For the radiation belt particle measurements, primarily we have used the observations from the twin spacecraft Van Allen Probes, which are in proximity to the equatorial plane(https://rbsp-ect.newmexicoconsortium.org/data_pub/rbspa/rept/level3/pitchangle/). For the current analysis, we have used only RBSP-A spacecraft data. The relativistic electron fluxes in differential energy channels have been obtained from the Relativistic Electron Proton Telescope (REPT) instrument (Baker et al., 2014) aboard Van Allen Probes. With 12 energy channels and 17 pitch angles, the REPT instrument provides highly resolved electron observations in terms of energy and pitch angle. Since their launch in 2012, the Van Allen Probes have observed a total of 13 HILDCAA events, which are summarized in Table 1. For this study, the relativistic electron flux data corresponding to these identified HILDCAA events has been sourced from the Van Allen Probe Science Gateway, available at https://rbspgway.jhuapl.edu/.

For the space-based ULF wave analysis during HILDCAA events, we utilized the level-2 data from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS), accessible through the Van Allen Probe Science Gateway at https://emfisis.physics.uiowa.edu/Flight/RBSP-A/L2/. Additionally, for the ground-based ULF wave analysis, we employed data from the IMAGE network of ground magnetometers, which can be found at https://space.fmi.fi/image/www/index.php?page=user_defined.

As mentioned in the introduction, geomagnetic storms, substorms, and interplanetary (IP) shocks are crucial drivers of the acceleration and variability of relativistic electron fluxes in the outer radiation belt (Reeves et al., 2003; Schiller et al., 2018; Bhaskar et al., 2021). In this study, we focus on examining the impact of High-Intensity Long-Duration Continuous Auroral Electrojet Activity (HILDCAA) events on relativistic electron behavior. Figure 1 displays the averaged data of interplanetary (IP) and geomagnetic parameters at both 1-hour and 1-minute resolutions for all 13 HILDCAA events. For each event, we considered a period extending 3 days before the start date and 7 days after, resulting in a consistent 10-day window for all events. Subsequently, we applied the superposed epoch analysis method to derive the averaged characteristics of the HILDCAA events. In this analysis, the onset of each HILDCAA event is indicated by the zero epoch.

This 1-hour resolution superposed epoched plot of the studied variables gives us a broader view of how IP and geomagnetic parameters varied during a typical HILDCAA event. For a closer look, we have also overplotted the 1-min resolution data in Fig. 1.

The figure illustrates the response of key parameters during the onset of the HILDCAA event. In the upper panel, the AU and AL indices reflect significant changes: the AE index shows a sharp increase while the AL index decreases, indicating intensified auroral currents. The presence of eastward electrojets is evident from the rise in the AU index. Before the event, the interplanetary magnetic field components (B_x, B_y, B_z) in GSM coordinates remain relatively stable, nu shows strong fluctuations as compared to pre-onset time. Following the zero epoch, a noticeable disturbance in B_z suggests that the coupling between the solar wind and the magnetosphere has intensified. Additionally, the strongly fluctuating solar wind motional electric field E indicates enhanced convection within the magnetosphere. After the onset of HILDCAA, solar wind speed V_sw increases, which is associated with prolonged geomagnetic activity linked to corotating interaction regions (CIRs). Solar wind density N_p and dynamic pressure P_sw also rise prior to the onset, contributing to magnetospheric compression and increased energy input. In contrast, the Sym-H index, which tracks the symmetric ring current, displays a decrease; however, this decline is not as pronounced as that observed during the main phase of a geomagnetic storm, remaining close to -30 nT, typical of geomagnetically quiet periods. Finally, the steady increase in proton temperature T_p after the zero epoch aligns with the presence of a sustained, high-energy solar wind stream.

In a study by Anderson et al. (2015), it was found that small storms can be just as effective as large storms in both enhancing and depleting electron fluxes in the Van Allen belts. The research revealed that the probability of flux enhancement increases with faster solar wind conditions, irrespective of the storm size. This study underscores the importance of examining cause-and-effect relationships in non-storm conditions to more accurately quantify electron acceleration and loss processes. This insight is particularly relevant when considering the impact of HILDCAA events, which, through prolonged and intense geomagnetic activity in high latitude, contribute to the observed enhancements in relativistic electron fluxes within the Van Allen belts though only weak ring current intensification present.

The present study investigated the impact of HILDCAAs on radiation belt electrons, specifically analyzing events that occurred during Van Allen Probes. The superposed epoch analysis shows delayed enhancements in relativistic electron fluxes, particularly at higher L-shells and in lower energy channels (below 5 MeV), with flux increases occurring 2-3 days after the event onset. Electron flux variations are observed to be pitch angle dependent, with greater energization for perpendicular particles than field-aligned ones. Both space and ground-based data confirm enhanced ULF wave power near the onset of HILDCAAs, particularly at higher L-shells. Time-lag correlations between solar wind speed and electron flux reveal that higher energy channels at higher L-shells experience shorter delays, suggesting complex spatial and energy-dependent electron acceleration processes during HILDCAAs.

Particle flux enhancement in the magnetosphere is primarily driven by mechanisms such as radial diffusion, wave-particle interactions, substorm injections, magnetospheric convection, and enhanced plasma wave activity (Reeves et al., 2003; Tsurutani et al., 2004; Baker et al., 2014; Roederer, 2012; Thorne et al., 2021; Tsurutani & Lakhina, 1997). High-speed solar wind streams (HSSs) play a critical role in energizing the magnetosphere, particularly by driving ULF wave activity, which transfers significant energy into the system and can lead to the energization of particles (Tsurutani & Lakhina, 1997). HILDCAA events are closely associated with ULF waves, making them key drivers of flux variations in the magnetosphere. ULF waves, with frequencies between 1 mHz and 1 Hz, interact with high-energy ions and electrons through drift and bounce resonances, particularly influencing particles with pitch angles near $90^{\circ}$. These waves affect radial diffusion processes, redistributing fluxes within the outer radiation belts and impacting relativistic electron fluxes (Pokhotelov et al., 2016). The gradual nature of radial diffusion (Lejosne & Kollmann, 2020) leads to delayed flux enhancements, consistent with the observed time lag between peak relativistic electron flux and solar wind speed. Additionally, particle acceleration seen in the current study shows a pitch angle dependence, with maximum energies closely tied to pitch angles. This strong pitch angle dependence underscores the critical role of ULF waves in accelerating relativistic electrons in the magnetosphere.

In contrast, VLF waves, which have frequencies between 3 kHz and 30 kHz, primarily interact with lower-energy electrons (in the keV range) through cyclotron resonance and Landau damping. Although VLF waves do not predominantly contribute to the acceleration of higher-energy particles as ULF waves do, they play a crucial role in scattering and redistributing lower-energy electrons across a range of pitch angles. This can lead to changes in the pitch angle distribution and can even result in particle precipitation into the atmosphere. Table  2 summarizes the basic difference between the ULF and VLF waves for the readers for easier understanding.

Our study further underscores that during HILDCAA events, ULF waves act as primary drivers of the particle flux variation due to their ability to resonate with higher-energy particles, while VLF waves, though important, primarily affect the low-energy particle population. This aligns with findings from earlier studies like Miyoshi et al. (2013); Hajra et al. (2024), which suggest that a southward interplanetary magnetic field (IMF) enhances conditions for whistler-mode chorus waves, thus accelerating electrons in the outer radiation belt. These wave-particle interactions are critical to understanding particle dynamics and flux variations during HILDCAA events, especially with ULF waves playing a dominant role.

This study investigates the variability of relativistic electron fluxes ($>1$ MeV) during HILDCAA events observed by NASA’s Van Allen Probes, which provided unprecedented high-resolution particle flux measurements. The analysis shows that flux enhancements occur between 0 to 2 days after the onset of HILDCAA events, with the highest energy flux observed reaching up to $\sim$4 MeV. Notably, electrons with perpendicular pitch angles exhibited greater flux enhancement and energization compared to field-aligned electrons, indicating that ULF waves play a key role in driving radial diffusion. The observed time delay in flux enhancement, ranging from several hours to days, further supports the involvement of ULF waves in the diffusion process. Additionally, the presence of Very Low Frequency (VLF) waves during HILDCAA events (Hajra et al., 2015b) may contribute to the acceleration of non-relativistic electrons, complementing the overall particle energization process.