High-intensity Long-duration Continuous AE Activity Events Research Articles

Enhancements of relativistic electron flux at geostationary orbit during high-intensity, long-duration, continuous AE activity (HILDCAA) from 2015 to 2017

This work characterizes the enhancements of relativistic electron flux (MeV) in the geostationary orbit (GEO) in High Intensity, Long Duration, Continuous AE Activity (HILDCAA) occurred during declining phase of solar cycle 24. We employed the relativistic electron (0.8 - 2.0 MeV) flux and low-energy electron (40 - 130 keV) flux measured by GOES-13 and POES satellites, respectively. Typically, the relativistic electron flux increases, while the low-energy electron flux decreases in the long recovery phase of moderate storms. The enhancements of E > 0.8 MeV and > 2.0 MeV occurred promptly and ∼1.0 day after the HILDCAA onset, respectively. A case study of short HILDCAA events shows that low solar wind dynamic pressure and long-lasting high amplitude Alfvén waves are efficient triggers of the relativistic electron enhancement at GEO. Large convection from magnetic reconnection in HILDCAA would induce substorms that injected more seed electrons for the acceleration. The peaks of the E > 2.0 MeV flux are more delayed than of the E > 0.8 MeV. After the onset of short- and prolonged-period HILDCAA, the peak flux of E > 0.8 MeV occurred about 2 and 4 days, respectively, and of E > 2.0 MeV occurred about 2 and 5 days, respectively.

The ionospheric response to high-intensity long duration continuous AE activity (HILDCAA) event (13–15 April 2005) over mid-latitude African region

The ionospheric responses to High-Intensity Long Duration Continuous Auroral Electrojet Activity (HILDCAA) event which happened following the CIR-driven storm were studied over the southern hemisphere mid-latitude in the African sector. The 13–15 April 2005 event was analysed to understand some of the mechanisms responsible for the ionospheric changes during HILDCAA event. The ionosonde critical frequency of F2 layer (foF2) and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) were used to analyse the ionospheric responses. The daytime increase in foF2 and TEC values were observed on 13 April 2005. The TEC and foF2 enhancement could be attributed to Large Scale Traveling Ionospheric Disturbances (LSTIDs), increase in thermospheric neutral composition changes, Prompt Penetration Electric Field (PPEF) and an expansion of Equatorial Ionization Anomaly (EIA) to the mid-latitude.

Wavelet analysis of the magnetotail response to solar wind fluctuations during HILDCAA events

Abstract. In this work a study of the effects of the high-intensity long-duration continuous AE activity (HILDCAAs) events in the magnetotail was conducted. The aim of this study was to search the main frequencies during HILDCAAs in the Bx component of the geomagnetic field in the magnetotail, as well as the main frequencies, at which the magnetotail responds to the solar wind during these events. In order to conduct this analysis the wavelet transform was employed during nine HILDCAA events that coincided with Cluster spacecraft mission crossing through the tail of the magnetosphere from 2003 to 2007. The most energetic periods for each event were identified. It was found that 76 % of them have periods ≤4 h. With the aim to search the periods that have the highest correlation between the IMF Bz (OMNI) component and the Cluster Bx geomagnetic field component, the cross wavelet analysis technique was also used in this study. The majority of correlation periods between the Bz (IMF) and Bx component of the geomagnetic field observed also were ≤4 h, with 62.9 % of the periods. Thus the magnetotail responds stronger to IMF fluctuations during HILDCCAS at 2–4 h scales, which are typical substorm periods. The results obtained in this work show that these scales are the ones on which the coupling of energy is stronger, as well as the modulation of the magnetotail by the solar wind during HILDCAA events.

High-intensity, long-duration, continuous AE activity events associated with Alfvénic fluctuations in 2003

Abstract. The interaction between a fast-speed and a low-speed stream causes large-amplitude Alfvénic fluctuations; consequently, the intermittency and the brief intervals of southward magnetic field associated with Alfvén waves may cause high levels of AE activity, the so-called high-intensity, long-duration, continuous AE activity (HILDCAA). In this article, the 4 h windowed Pearson cross-correlation (4WPCC) between the solar wind velocity and the interplanetary magnetic field (IMF) components is performed in order to confirm that the less strict HILDCAA (HILDCAAs*) events include a larger number of Alfvén waves than the HILDCAA events, once HILDCAAs disregard part of the phenomenon. Actually, a HILDCAA event is entirely contained within a HILDCAA* event. However, the opposite is not necessarily true. This article provides a new insight, since the increase of Alfvén waves results in an increase of auroral electrojet activity; consequently, it can cause HILDCAAs* events. Another important aspect of this article is that the superposed epoch analysis (SEA) results reaffirm that the HILDCAAs* are associated with high-speed solar streams (HSSs), and also the HILDCAAs* present the same physical characteristics of the traditional HILDCAA events.

Impacts on Cosmic-Ray Intensity Observed During Geomagnetic Disturbances

Geomagnetic disturbances are the results of interplanetary causes such as high-speed streamers (HSSs), interplanetary coronal mass ejections (ICMEs), corotating interaction regions (CIRs), and magnetic clouds. During different forms of geomagnetic disturbances, we observed changes in the count rate at neutron monitors that are kept at various locations. We studied the count rates measured by neutron monitors at four stations at various latitudes during different categories of geomagnetic events and compared them. We analysed five events: a geomagnetically quiet event, a non-storm high-intensity long-duration continuous AE activity (HILDCAA) event, a storm-preceded HILDCAA event, a geomagnetic substorm event, and a geomagnetic moderate storm event. We based our analysis on geomagnetic indices, solar wind parameters, and interplanetary magnetic field (IMF) parameters. We found that the strength of the modulation was least during the quiet event and highest during the storm-preceded HILDCAA. By analysing the cause of these geomagnetic disturbances, we related each decrease in the neutron monitor data with the corresponding solar cause. For the ICME-driven storm, we observed a decrease in neutron monitor data ranging from 6% to 12% in all stations. On the other hand, we observed a decrease ranging from 2% to 5% for the HSS-driven storm. For the non-storm HILDCAA, we observed a decrease in neutron monitor data of about 1% to 1.5%. For the quiet event, the neutron monitor data fluctuated such that there was no overall decrease in all stations.

A study on the main periodicities in interplanetary magnetic field Bz component and geomagnetic AE index during HILDCAA events using wavelet analysis

The interplanetary and geomagnetic characteristics of High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events are studied using wavelet analysis technique. The Morlet wavelet transform was applied to the 1min interplanetary magnetic field (IMF) Bz component and the geomagnetic AE index during HILDCAA events. We have analyzed the AE data for the events occurring between 1975 and 2011, and the IMF Bz data (both in GSE and GSM) for the events between 1995 and 2011. We analyzed the scalograms and the global wavelet spectrum of the parameters. For 50% of all HILDCAA events, the main periodicities of the AE index are generally between 4 and 12h. For the Bz component, the main periodicities were found to be less than 8h for ~56% of times in GSM system and for ~54% of times in GSE system. It is conjectured that the periodicities might be associated with the Alfvén waves which have typical periods between 1 and 10h. The results are discussed in the light of self organized criticality theory where the physical events have the capacity of releasing a considerable amount of energy in a short interval of time.

Relativistic electron acceleration during HILDCAA events: are precursor CIR magnetic storms important?

We present a comparative study of high-intensity long-duration continuous AE activity (HILDCAA) events, both isolated and those occurring in the “recovery phase” of geomagnetic storms induced by corotating interaction regions (CIRs). The aim of this study is to determine the difference, if any, in relativistic electron acceleration and magnetospheric energy deposition. All HILDCAA events in solar cycle 23 (from 1995 through 2008) are used in this study. Isolated HILDCAA events are characterized by enhanced fluxes of relativistic electrons compared to the pre-event flux levels. CIR magnetic storms followed by HILDCAA events show almost the same relativistic electron signatures. Cluster 1 spacecraft showed the presence of intense whistler-mode chorus waves in the outer magnetosphere during all HILDCAA intervals (when Cluster data were available). The storm-related HILDCAA events are characterized by slightly lower solar wind input energy and larger magnetospheric/ionospheric dissipation energy compared with the isolated events. A quantitative assessment shows that the mean ring current dissipation is ~34 % higher for the storm-related events relative to the isolated events, whereas Joule heating and auroral precipitation display no (statistically) distinguishable differences. On the average, the isolated events are found to be comparatively weaker and shorter than the storm-related events, although the geomagnetic characteristics of both classes of events bear no statistically significant difference. It is concluded that the CIR storms preceding the HILDCAAs have little to do with the acceleration of relativistic electrons. Our hypothesis is that ~10–100-keV electrons are sporadically injected into the magnetosphere during HILDCAA events, the anisotropic electrons continuously generate electromagnetic chorus plasma waves, and the chorus then continuously accelerates the high-energy portion of this electron spectrum to MeV energies.

RELATIVISTIC (E> 0.6, > 2.0, AND > 4.0 MeV) ELECTRON ACCELERATION AT GEOSYNCHRONOUS ORBIT DURING HIGH-INTENSITY, LONG-DURATION, CONTINUOUS AE ACTIVITY (HILDCAA) EVENTS

Radiation-belt relativistic (E > 0.6, > 2.0, and > 4.0?MeV) electron acceleration is studied for solar cycle 23 (1995-2008). High-intensity, long-duration, continuous AE activity (HILDCAA) events are considered as the basis of the analyses. All of the 35 HILDCAA events under study were found to be characterized by flux enhancements of magnetospheric relativistic electrons of all three energies compared to the pre-event flux levels. For the E > 2.0?MeV electron fluxes, enhancement of >50% occurred during 100% of HILDCAAs. Cluster-4 passes were examined for electromagnetic chorus waves in the 5 0.6, > 2.0, and > 4.0?MeV electrons occurred ~1.0?day, ~1.5?days, and ~2.5?days after the statistical HILDCAA onset, respectively. The statistical acceleration rates for the three energy ranges were ~1.8?? 105, 2.2?? 103, and 1.0?? 101 cm?2 s?1 sr?1 d?1, respectively. The relativistic electron-decay timescales were determined to be ~7.7, 5.5, and 4.0?days for the three energy ranges, respectively. The HILDCAAs were divided into short-duration (D ? 3?days) and long-duration (D > 3?days) events to study the dependence of relativistic electron variation on HILDCAA duration. For long-duration events, the flux enhancements during HILDCAAs with respect to pre-event fluxes were ~290%, 520%, and 82% for E > 0.6, > 2.0, and > 4.0?MeV electrons, respectively. The enhancements were ~250%, 400%, and 27% respectively, for short-duration events. The results are discussed with respect to the current understanding of radiation-belt dynamics.

Solar wind‐magnetosphere energy coupling efficiency and partitioning: HILDCAAs and preceding CIR storms during solar cycle 23

AbstractA quantitative study on the energetics of the solar wind‐magnetosphere‐ionosphere system during High‐Intensity, Long‐Duration, Continuous AE Activity (HILDCAA) events for solar cycle 23 (from 1995 through 2008) is presented. For all HILDCAAs, the average energy transferred to the magnetospheric/ionospheric system was ~6.3 ×1016 J, and the ram kinetic energy of the incident solar wind was ~7.1 ×1018 J. For individual HILDCAA events the coupling efficiency, defined as the ratio of the solar wind energy input to the solar wind kinetic energy, varied between 0.3% and 2.8%, with an average value of ~0.9%. The solar wind coupling efficiency for corotating interaction region (CIR)‐driven storms prior to the HILDCAA events was found to vary from ~1% to 5%, with an average value of ~2%. Both of these values are lower than the> 5% coupling efficiency noted for interplanetary coronal mass ejection (and sheath)‐driven magnetic storms. During HILDCAAs, ~67% of the solar wind energy input went into Joule heating, ~22% into auroral precipitation, and ~11% into the ring current energy. The CIR‐storm Joule heating (~49%) was noticeably less than that during HILDCAAs, while the ring current energies were comparable for the two. Joule dissipation was higher for HILDCAAs that followed CIR‐storms (88%) than for isolated HILDCAAs (~60%). Possible physical interpretations for the statistical results obtained in this paper are discussed.

Solar cycle dependence of High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) events, relativistic electron predictors?

AbstractHigh‐Intensity, Long‐Duration, Continuous AE Activity (HILDCAA) events are studied using long‐term geomagnetic and solar wind/interplanetary databases. We use the strict definition of a HILDCAA event, that it occurs outside of the main phase of a magnetic storm, the peak AE is >1000 nT, and the duration is at least 2 days long. One hundred thirty‐three events have been identified from the AE indices in the 1975 to 2011 interval, a ~3½ solar cycle span. Of the 133 events, 99 had simultaneous interplanetary data available. The overwhelmed majority (94%) of these latter cases were associated with high‐speed solar wind stream (HSS) events. The remaining 6% of the cases occurred after the passage of interplanetary coronal mass ejections (ICMEs). The HSS‐related events were typically associated with large interplanetary magnetic field (IMF) Bz variances. The ICME‐related events were characterized by steady southward Bz intervals or low‐frequency fluctuations, both of which we view as possible different interplanetary phenomena. HILDCAA events have been found to have their largest occurrence frequency in the solar cycle descending phase (~6.8/year) with the second largest at solar minimum (~3.5/year). The occurrence frequencies were considerably lower in the ascending phase (~2.5/year) and at solar maximum (~2.2/year). Thus, HILDCAAs can occur during all phases of the solar cycle, with the descending phase approximately three times more likely to have an event than at solar maximum and the ascending phase. The HILDCAA events that occurred in the declining phase and at solar minimum were >20% longer in duration than those in the ascending phase and solar maximum, respectively. The events during the recent solar and geomagnetic minima, 2007–2009, were, on the average, ~17% and 14% weaker in peak AE than the events during the previous two minima of 1995–1997 and 1985–1987, respectively. The recent minimum events were ~35% and 41% shorter in durations, respectively, than the events during those previous minima. The yearly occurrence of the events exhibited statistically significant correlation (>0.70) with yearly average speed and number of HSSs. No seasonal dependence of HILDCAA was noted.

Are repetitive particle injections during high‐speed solar wind streams classic substorms?

The relation between substorm occurrences, energetic particle injections, and High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) events has been an issue. To understand their relationship, we use global auroral images from the Wideband Imaging Camera (WIC) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft to investigate auroral activity near the times of repetitive particle injections that occur every ∼0.5 to 4 h during HILDCAA events. For the examined HILDCAA intervals during January–July 2003, we identified a total of 481 injection events. Good WIC imaging was available for 121 of these and showed substorm onset brightenings for a significant fraction of the 121 injections. Specifically, excluding 16 injections with ambiguous timings owing to the lack of a near‐midnight spacecraft, we find that 93 of 105 injections (∼88%) occurred within ±5 min of a substorm auroral onset. If we include cases with up to 10‐min timing difference, the percentage becomes 97%. These results indicate that a significant fraction of HILDCAA‐time repetitive particle injections are substorm‐associated and thus suggest that substorms are a major component of HILDCAA geomagnetic activity and a possible contributor to the sustained small negative Dst during these periods.

Equatorial ionospheric responses to high‐intensity long‐duration auroral electrojet activity (HILDCAA)

The knowledge of the coupling processes between the magnetosphere and the equatorial ionosphere is of basic importance to the understanding of the near‐Earth space weather. This study focuses on observational results of such coupling processes based on data collected during the phenomenon defined as high‐intensity long‐duration continuous AE activity (HILDCAA) which occurs outside the main phase of geomagnetic storms. The fact that the responses of the equatorial/low‐latitude ionosphere to HILDCAA events have not been specifically focused so far is one of the motivations for this study. Ionosonde data on hmF2, h′F, and foF2 from three locations in Brazil (magnetic equatorial station São Luis (SL), subequatorial station Fortaleza (FZ), and low‐latitude station Cachoeira Paulista (CP)) are analyzed together with ACE satellite data on solar wind and interplanetary magnetic and electric fields during three HILDCAA events that occurred in the years 2000 and 2001. The results did not indicate any presence of penetrating electric field disturbance during these events. However, they provided clear evidence of disturbance dynamo electric field and disturbance thermospheric winds, through F layer height changes that were similar but generally less intense than those observed during a typical storm event. The foF2 presented no particular disturbances that can be clearly attributed to the HILDCAA event. Previous extensive studies carried out by the authors on ionospheric storm effects for these same three stations clearly illustrate the much more intense F layer storm disturbances compared with HILDCAA events disturbances.

Evidence for particle injection as the cause of Dst reduction during HILDCAA events

The Earth's magnetic field at the equator, as monitored by the D st index, can stay below its quiet day value for days. This can happen after storms resulting in a very slow recovery of the D st index, or it can happen in the absence of a storm. Such “anomalous” behavior of the D st recovery is observed during times showing continuous auroral activity called High Intensity Long Duration Continuous AE Activity (HILDCAA) (Planet. Space Sci. 35 (1987) 405). The D st is mostly attributed to the ring current (RC), but it also depends on other current systems in the magnetosphere such as magnetopause, tail and auroral currents. It is well established that the magnetic effect of the RC is proportional to the energy content of the charged particles generating it (J. Geophys. Res. 64 (1959) 2239; 71 (1966) 3125). It is thus of interest to determine if the reduction of the D st during HILDCAAs is due to an increased RC or/and can be accounted for by an increase or a relocation of other current systems. This paper considers the injection of electrons and protons into the auroral and subauroral zone for four cases exhibiting HILDCAA activity. The total energy flux of the ions into the midnight/evening quadrant gives a good estimate of the energy injection into the RC (J. Geophys. Res. 107 (2002) 149). This injection, if it occurs during the recovery phase of a storm, prolongs the final decay of the D st to quiet day values, and if it occurs during times without storms the injection can maintain D st at more or less constant negative values for days. The ion injection into the RC is sufficient to account for the reduced D st index during HILDCAAs. It is determined that the HILDCAA events are associated with a low-level injection of protons into the outer portion of the RC above L equal 4. The HILDCAA events are thus not due to plasma sheet current intensifications or earthward motion of this current. The prolonged low-level ion injection is associated with fluctuations in the B z component of the solar wind magnetic field giving rise to magnetic field line merging on the front side of the magnetosphere (Planet. Space Sci. 35 (1987) 405). The duration of the B z negative phases are not long enough to drive a magnetic storm.

Interplanetary origin of geomagnetic activity in the declining phase of the solar cycle

Interplanetary magnetic field (IMF) and plasma data are compared with ground‐based geomagnetic Dst and AE indices to determine the causes of magnetic storms, substorms, and quiet during the descending phase of the solar cycle. In this paper we focus primarily on 1974 when the AE index is anomalously high . This year is characterized by the presence of two long‐lasting corotating streams associated with coronal holes. The corotating streams interact with the upstream low‐velocity (300–350 km s−1), high‐density heliospheric current sheet (HCS) plasma sheet, which leads to field compression and ∼ 15‐ to 25‐nT hourly average values. Although the Bz component in this corotating interaction region (CIR) is often < −10 nT, typically the field directionality is highly variable, and large southward components have durations less than 3 hours. Thus the corotating stream/HCS plasma sheet interaction region can cause recurring moderate (−100 nT ≤ Dst ≤ −50 nT) to weak (−50 nT ≤ Dst ≤ −25 nT) storms, and sometimes no significant ring current activity at all (Dst > −25 nT). Storms of major (Dst ≤ −100 nT) intensities were not associated with CIRs. Solar wind energy is transferred to the magnetosphere via magnetic reconnection during the weak and moderate storms. Because the Bz component in the interaction region is typically highly fluctuating, the corresponding storm main phase profile is highly irregular. Reverse shocks are sometimes present at the sunward edge of the CIR. Because these events cause sharp decreases in field magnitude, they can be responsible for storm recovery phase onsets. The initial phases of these corotating stream‐related storms are caused by the increased ram pressure associated with the HCS plasma sheet and the further density enhancement from the stream‐stream compression. Although the solar wind speed is generally low in this region of space, the densities can be well over an order of magnitude higher than the average value, leading to significant positive Dst values. Since there are typically no forward shocks at 1 AU associated with the stream‐stream interactions, the initial phases have gradual onsets. The most dramatic geomagnetic response to the corotating streams are chains of consecutive substorms caused by the southward components of large‐amplitude Alfvén waves within the body of the corotating streams. This auroral activity has been previously named high‐intensity long‐duration continuous AE activity (HILDCAAs). The substorm activity is generally most intense near the peak speed of the stream where the Alfvén wave amplitudes are greatest, and it decreases with decreasing wave amplitudes and stream speed. Each of the 27‐day recurring HILDCAA events can last 10 days or more, and the presence of two events per solar rotation is the cause of the exceptionally high AE average for 1974 (higher than 1979). HILDCAAs often occur during the recovery phase of magnetic storms, and the fresh (and sporadic) injection of substorm energy leads to unusually long storm recovery phases as noted in Dst. In the far trailing edge of the corotating stream, the IMF amplitudes become low, <3 nT, and there is an absence of large‐amplitude fluctuations (Alfvén waves). This is related to and causes geomagnetic quiet. There were three major (Dst ≤ −100 nT) storms that occurred in 1974. Each was caused by a nonrecurring moderate speed stream led by a fast forward shock. The mechanisms for generating the intense interplanetary Bs which were responsible for the subsequent intense magnetic storms was shock compression of preexisting southwardly directed Bz (Bs) for the two largest events and a magnetic cloud for the third (weakest) event. Each of the three streams occurred near a HCS crossing with no obvious solar optical or X ray signatures. It is speculated that these events may be associated with flux openings associated with coronal hole expansions. In conclusion, we present a model of geomagnetic activity during the descending phase of the solar cycle. It incorporates storm initial phases, main phases, HILDCAAs, and geomagnetic quiet. It uses some of the recent Ulysses results. We feel that this model is sufficiently developed that it may be used for predictions, and we encourage testing during the current phase of the solar cycle.

The cause of high-intensity long-duration continuous AE activity (HILDCAAs): Interplanetary Alfvén wave trains

It is shown that high intensity ( AE > 1,000 nT), long duration ( T > 2d) continuous auroral activity (HILDCAA) events are caused by outward (from the sun) propagating interplanetary Alfvén wave trains. The Alfvén waves are often (but not always) detected several days after major interplanetary events, such as shocks and solar wind density enhancements. Presumably magnetic reconnection between the southward components of the Alfvén wave magnetic fields and magnetospheric fields is the mechanism for transfer of solar wind energy to the magnetosphere. If the stringent requirements for HILDCAA events are relaxed, there are many more AE events of this type. A brief inspection indicates that these are also related to interplanetary Alfvénic fluctuations. We therefore suggest that most auroral activity may be caused by reconnection associated with Alfvén waves in the interplanetary medium.