Energetic Electron and Proton Interactions with Pc5 Ultra Low Frequency (ULF) Waves during the Great Geomagnetic Storm of 15–16 July 2000

The first time interval (0700–0820 UT) shows strong evidence of a global oscillation with frequency of 1.7 mHz (600 s) in the ground-based magnetometer stations except stations located near midnight such as IQA, PBQ, and RABB. Pc5 ULF waves with a frequency of 1.7 mHz appeared even at very low latitudes such as the MMB stations located at L ~1.6. The Faroes (FAR) station located near the dawn flank showed multiple discrete frequency waves with periods of 300 s (3.3 mHz) and 600 s (1.7 mHz). The largest amplitude toroidal waves with a frequency of 1.7 mHz, assuming a 90° ionospheric rotation into the the H-component on the ground, appear near the dusk flank (DAWS, CMO, BRW) while no clear oscillation appears near midnight.

The second time interval (0850–1010 UT) shows a spectral peak at 1.4 mHz (690 s) in BRW, ZKY, and CHD stations located near dusk. These longer period waves appear in the dayside of the magnetosphere during this time interval, and the largest amplitude still appears at the dusk flank. These waves are localized in the afternoon sector. However, GOES 8 which was located close to the dawn flank shows evidence of higher frequency (3.3 mHz) waves.

The third time interval (1010–1130 UT) shows that waves have the largest amplitude near dusk (BRW, ZYK, and CHD) with frequency of 1.2 mHz (810 s). Waves with frequency of 3.3 mHz (300 s) still appear in the dawn flank.

During the fourth time interval (1130–1250 UT), 1.7 mHz (600 s) waves were observed in the afternoon local time sector with the largest amplitude near dusk. Again, 3.3 mHz waves are still observed in the dawn flank (such as PBQ, GOES 8, GILL, and RABB stations).

In summary, multiple packets of discrete frequency Pc5 ULF waves appeared globally with a more limited local time extent during the great geomagnetic storm of July 16, 2000. Global Pc5 pulsations with a frequency of 1.7 mHz appeared in the dayside of the magnetosphere simultaneously, with the largest amplitude near dusk and pre-midnight. Also, waves with frequencies of 1.4 mHz (690 s) and 1.2 mHz (810 s) appeared localized from post-noon to pre-midnight with the largest amplitude near dusk. However, 3.3 mHz (300 s) waves appeared locally in the morning sector with the largest amplitude at the dawn flank. We also investigated particle flux measurements obtained from the Los Alamos National Laboratory (LANL) geosynchronous SOPA. Particle flux measurements obtained from the SOPA instrument on board the LANL geosynchronous satellites show evidence of an energetic particle and ULF wave interaction during the July 16 storm and this will be studied in the following section.

The first time interval (0700–0820 UT) is characterized as a global oscillation of 1.7 mHz frequency Pc5 waves in the ground-based magnetometer data and 3.3 mHz frequency Pc5 waves in the morning sector [see Fig. 3, interval (a)]. We observed at LANL 1994 and LANL 1989 that the proton and electron flux oscillate with the same frequency 1.7 mHz and that all energy channels of both electrons and protons oscillated in phase. In Fig. 2, a dotted vertical line in LANL 1994-084 during interval (a) is drawn to indicate that there was no phase change between electrons and protons and between different energies. On the other hand, LANL 1991- 080 shows 600 s (1.7 mHz) period flux modulation in the energy channels between 500 and 750 keV for electrons, and a 300 s (3.3 mHz) period modulation appears in the lower energy channels with a smaller amplitude. Proton flux oscillations at 1.7 mHz appear at most energies but with small amplitudes. The particle flux modulations seem to be related with magnetospheric Pc5 waves observed on the ground and in space. Both particle flux and geomagnetic field oscillations at 1.7 mHz show the largest amplitude in the afternoon. For example, CMO [L = 5.72, magnetic local time (MLT) = 19:49–21:09] shows the largest amplitude pulsations on the dusk flank and electron flux and proton flux oscillate simultaneously with very large amplitude at LANL 1994-084 (L = 6.8, MLT = 13:42–14:58) and LANL 1989-046 (L = 6.6, MLT = 19:46–21:02). In the same manner, 3.3 mHz frequency waves appeared in the morning for both magnetic field from FAR station (L = 4.3, MLT = 06:44–08:04) and electron flux from LANL 1991-080 (L = 6.6, MLT = 07:21– 08:37). Coherent observations of Pc5 pulsations and particle flux oscillations with the same frequency at the same time interval strongly suggest that there were interactions between Pc5 ULF waves and energetic particles.

The second time interval (0850–1010 UT) [Fig. 3(b)] shows magnetic pulsations with a peak amplitude at 1.4 mHz at ZKY (L = 4.04, MLT = 16:11–17:31) located near dusk. Clear phase and amplitude change across the energy channels appear in the observed electron and proton flux modulations (see the dotted lines during interval (b) on LANL 1994- 084 flux data in Fig. 2 for a clear example). Both proton and electron flux modulation appeared at LANL 1994-084 (L = 6.8, MLT = 15:26–16:43), but only electron modulation appeared in LANL 1989-046 (L = 6.6, MLT = 21:31–22:48). LANL 1991-080 (L = 6.6, MLT = 09:06–10:23) also shows only electron oscillation at 1.4 mHz with small amplitude in the energy channels between 315 and 750 keV. The species and MLT dependence of modulation can be explained by the drift path of electron and proton as well as the location of satellites. A detailed explanation of the observed modulation will be presented in the discussion section.

The third time interval (1010–1130 UT) [Fig. 3(c)] is characterized by magnetic Pc5 waves with power spectra with a discrete peak at 1.2 mHz, e.g., ZYK (L = 4.04, MLT = 17:31–18:51) and CMO (L = 5.72, MLT = 22:59–00:19). Phase and amplitude changes across energy channels are clear in electron flux. Large amplitude electron flux modulation with 1.2 mHz frequency appeared in LANL 1994-084 (L = 6.8, MLT = 16:43–18:01) and LANL 1989-046 (L = 6.6, MLT = 22:48–00:06) while proton flux modulation didn’t appear at either satellite (see Fig. 3). On the other hand, LANL 1991- 080 which was located in the morning sector observed 1.2 mHz frequency electron flux oscillation at higher energy channels between 315 and 750 keV and 3.3 mHz electron flux oscillation at lower energy channels.

The fourth time interval (1130–1250 UT) [Fig. 3(d)] shows magnetic power spectra with a spectral peak at 1.7 mHz in KIL (L = 6.09, MLT = 08:48–10:08) station located in the post-noon sector. Electron flux modulation appeared in both LANL 1994-084 (L = 6.8, MLT = 18:01–19:22) and LANL 1989-046 (L = 6.6, MLT = 00:06–01:27) with the largest amplitude in the energy channel at around 500–750 keV. On the other hand, proton flux modulations appeared only in LANL 1994-084 with small amplitudes. Similar to the previous interval, LANL 1991-080 still observes 1.7 mHz electron flux oscillations at higher energy channels and 3.3 mHz oscillations at lower energy channels.

The top row of Fig. 5 shows the electron and proton drift paths. This figure shows the drift paths of energetic electrons and protons with different initial energy at 0800 UT (top) on 16th July 2000. Particles are launched with initial energies of 100 keV, 200 keV, 300 keV, and 400 keV at L = 6.6 and in the afternoon sector at 1800 LT and drift around a static dipole magnetic field model. In Fig. 5 top panel, the electrons drift on closed orbits but protons drift on open trajectories for the lower energies (100 and 200 keV). The location of the LANL geosynchronous satellites and the approximate MLT extent of the observed ULF waves are also shown. LANL 1994-094 was located around 1630 MLT and LANL 1989-046 was located near 2230 MLT at this time. 1.7 mHz ULF waves (black solid oscillation) were observed globally and 3.3 mHz waves (red solid oscillation) were observed in the dawn flank. The arrows indicate the drift directions of electrons (eastward) and protons (westward). During 0700–0820 UT, the ground-based magnetometers showed global waves of 1.7 mHz and modulations of energetic electron and proton flux were also observed in all three LANL satellites at the same frequency. As mentioned in section 4, there was no phase change between electrons and protons and between different energies. A possible explanation for this observed flux modulation might be the advection of an energetic particle density gradient across the satellites. If there is a gradient in the particle density in the direction of wave magnetic perturbations and the field line displacement, flux oscillation can result (e.g., Southwood 1973;Southwood & Kivelson 1981). In the case of advection, as indicated by the dotted line in Fig. 2, both energetic electron and proton flux oscillated in phase across all of the different energy channels.

In the morning sector, the ground-based magnetometers observed a 3.3 mHz frequency oscillation as well as a 1.7 mHz frequency oscillation (see Fig. 2, FAR station). LANL 1991-080, located in the morning sector, observed 1.7 mHz electron flux pulsations at higher energy channels and 3.3 mHz electron flux oscillations at lower energy channels. On the other hand, the proton flux only oscillated at 1.7 mHz. One possible explanation for this is that a westward traveling proton would be detected at the LANL 1991-080 satellite before the proton interacted with the 3.3 mHz wave oscillations further to the west (see Fig. 5, top panel). To observe the flux modulation at the satellite, the energetic particles must pass through the wave before they reach the satellite.

During 0850–1010 UT, 1.4 mHz ULF waves were observed on the ground near noon and in the afternoon sector from the ground-based magnetometers. Large amplitude waves were observed near dusk (cf. ZYK and CHD stations). At the same time, proton and electron modulations clearly appeared at LANL 1994-084 at 15:26–16:43 MLT while only electron flux modulation appeared at LANL 1989-046 at 21:31-22:48 MLT (see Fig. 2). During this second interval, typical resonance characteristics appear in the flux observations such as a phase change and amplitude peak across the resonant energy (315–500 keV for electrons and 175–250 keV for protons) (see Fig. 2). Thus, we suggest that both electrons and protons were modulated by a resonant interaction with Pc5 ULF waves. From the observations, the 1.4 mHz proton oscillation shows largest amplitude at energies of 175–250 keV at L = 6.6. For an electron moving eastward around the Earth, electrons could be modulated by poloidal mode ULF waves which have azimuthal electric field components. However in a compressed dipole, the toroidal mode can also accelerate electrons via drift-resonance interaction (Elkington et al. 1999;2003). The resonance condition for drift-resonant acceleration in a compressed dipole is

where ω, ω_d and m represent wave frequency, drift frequency and azimuthal wave number, respectively. By solving equation 5, we can find out that the resonant azimuthal wave number (m) for this wave is around –3 (–solution) or –5 (+solution) for proton modulation. Electron flux modulation also has its largest amplitude near energies of 315–500 keV at L = 6.6 which gives a resonant azimuthal wave number around 3 or 1. If the ULF waves are standing in the azimuthal direction, these could have both positive and negative signs of azimuthal wave number (m) at the same time. If this is the case, then both electrons and protons could be resonant with the ULF waves at the same time. Thus, satellites can observe proton modulation arising from their interaction with westward traveling ULF waves and electron flux can be modulated by eastward traveling ULF waves. This means there should be waves propagating in both directions. We can find evidence of azimuthal standing waves, with components traveling both westward and eastward, in the magnetic field data observed by the ground-based magnetometers. Fig. 4 shows H-component magnetic fields observed at different local times with similar latitudes. From top to bottom, the panels in this plot show magnetic field H-component variations from BEL (CGM latitude 47.52, CGM longitude 96.19) and HAD (47.69, 74.86), HER (–42.05, 82.6) and AMS (–49.06, 138.84), and BMT (34.48, 188.71) and MMB (37.05, 215.35) in CGM coordinates. No clear phase delay appears between the stations located at different longitudes but at similar latitudes. Hence, we infer that there were both westward and eastward traveling waves creating an azimuthal standing wave as shown in Fig. 4. Under the condition of both westward and eastward traveling waves, creating an azimuthally standing mode structure, we suggest that the modulation of electrons and protons can both be observed simultaneously as seen here. The absence of proton modulation in LANL 1989-046 can be explained due to the relative location of the satellite with respect to the drift path of protons. Since the LANL 1989- 046 satellite is located on the eastern side of the ULF waves, the modulated protons will be observed after protons circle all around the Earth on their drift orbit. However, lower energy protons will drift on open trajectories as shown in the right panel of Fig. 5 (bottom panel), such that proton modulation at these energies will not be detected since the protons do not return to the satellite location. Also, since the drift paths are not symmetric and reach different distances from the Earth especially between dawn and dusk, there are chances protons modulated by waves will not be detected at satellites at fixed geosynchronous altitudes and which are located far from the source.

During 1010–1130 UT, ULF waves with 1.2 mHz frequency were observed in the afternoon sector with largest amplitude near the dusk flank (see Fig. 6, top panel). The locations of LANL 1994-084, LANL 1989-046 and LANL 1991-080 are 1720 MLT, 2330 MLT, and 1100 MLT at 1030 UT, respectively. Energetic electron flux modulation at the frequency of 1.2 mHz was seen in LANL 1994-084 and LANL 1989-046 very clearly, but no proton flux modulations were observed at either of these satellites. The reasons for the absence of proton modulations may be related to the wave propagation direction. ULF waves propagating eastward interact only with electrons not with protons in drift resonance. Also, the proton’s open drift trajectory may obstruct the return of modulated protons to the satellite location. Electron flux modulation observed at LANL 1994- 084 is much larger in amplitude than at LANL 1989-046. This might be due to a particle injection in the low energy channel being superposed on top of flux modulations for LANL 1994-084 satellites detector. Drift-resonant behavior was observed during this interval in the electron enhanced flux oscillation of 1.2 mHz with a frequency and maximum amplitude at energies of 105–225 keV at L = 6.6, such a wave would be resonated with the electrons if the wave had azimuthal wave number of m = 2 or m = 4 (m values were determined by solving equation (5)). Thus, we suggest that the electron flux modulation is due to a drift resonance with the Pc5 ULF waves.

During 1130–1250 UT, magnetic field perturbations of 1.7 mHz frequency were observed in the afternoon sector. The location of LANL 1994-084 and LANL 1989-046 were 1820 MLT and 0030 MLT at 1130 UT, respectively (see Fig. 6, bottom panel). Electron flux modulations were also observed at both satellites but more of larger amplitude at LANL 1994- 084 which was located closer to the wave source. Similar to the second interval [interval (b)], a proton flux modulation was detected only at LANL 1994-084. LANL 1994-084 may have observed a proton flux modulation as a result of the satellite being located in the center of the region of wave activity. In this case, the flux modulation can be explained using the same reasoning as discussed for the absence of proton modulation in the interval (b).