Ching-Chang Cheng

Spectral power of low-latitude Pi 2 pulsations at the 210° magnetic meridian stations and plasmaspheric cavity resonances

Pi 2 pulsations at low latitudes are examined with magnetic fields data at the 210° magnetic meridian (MM) stations in 1991. Due to high degree of coherence over most latitudes, 68 low-latitude Pi 2 events at the 210° MM stations are identified with reference to same waveforms on the magnetogram at Lunping (189.5° MM, L = 1.06). With spectral power analysis, the ratio of the first four harmonic frequencies of low-latitude Pi 2 pulsations is about 1: (1.7 ± 0.1): (2.3 ± 0.1): (2.9 ± 0.1). By using abox model for the inner magnetosphere, the cold linearized MHD wave equations are studied with realistic Alfven speed profile for nonuniform ambient magnetic fields. With appropriate parameters to depict the magnetospheric environments during the aforementioned period, numerical results are acquired with the fourth order Runge-Kutta method. It is found that the ratio of the first four harmonic frequencies of plasmaspheric cavity resonances is about 1: 1.7: 2.4: 3.1 that is consistent with data analysis. This suggests that low-latitude Pi 2 pulsations at the 210° MM stations may be plasmaspheric cavity resonances driven by fast compressional waves owing to the impulsive source at the magnetotail.

Evidence of the coupling of a fast magnetospheric cavity mode to field line resonances

We present an evidence of the coupling of a fast magnetospheric cavity mode to field line resonances. The power of filtered H and D components of Pi 2 pulsations during March–April in 1978 and 1979 from IGS chain in the United Kingdom (UK) shows that there are two enhancements located near L = 3.5 and 5.5 in the distribution profile. The irreversible hydromagnetic wave coupling is studied by using a box model for the magnetosphere. The cold linearized MHD equations is examined with realistic Alfven speed profile for the uniform and nonuniform ambient magnetic fields, respectively. With appropriate parameters to depict the magnetospheric environments during the aforementioned period, numerical results are acquired with the fourth order Runge-Kutta method and quite consistent with data analysis. We suggest that the coupling of a fast magnetospheric cavity mode driven by fast compressional waves owing to the impulsive source at the magnetotail to resonant field lines at low latitudes is a possible scenario of the propagation mechanism for low latitude Pi 2 pulsations.

On consecutive bursts of low-latitude Pi2 pulsations

Abstract Consecutive bursts of low-latitude Pi2 pulsations are examined with magnetic field data obtained by the Sino Magnetic Array at Low Latitudes (SMALL) in 1999. To ascertain the relationship with the substorm onset, 33 events consisting of two consecutive Pi2 bursts are selected from the observations at the Wuhan and Beijing stations, with reference to H-component magnetic bays in high-latitude magnetograms. The dominant frequencies of these consecutive Pi2 bursts are mostly in the range 10– 20 mHz . The frequencies of the two Pi2 bursts are less correlated with each other at the lower latitude station, Wuhan, than at Beijing. The dominant frequency increases as the Kp index increases. Observations with ACE and Wind satellite show that the first Pi2 burst occurs significantly after the southward turning of the interplanetary magnetic field (IMF) and the second Pi2 often occurs shortly after a northward turning of the IMF. Thus, the first Pi2 burst is not a signature of the beginning of the flux transfer to the tail but perhaps signals the initial onset of reconnection in the tail. The interplanetary data can be used to estimate the amount of reconnected magnetic flux transported to the tail. The delay time of two consecutive bursts of low-latitude Pi2 is correlated with this estimated flux pileup. If the second burst of Pi2 signals the onset of reconnection of the open field lines in the tail lobes, then this observation implies that the rate of reconnection of the closed field lines in the plasma sheet is more rapid the greater is the rate of reconnection at the nose. Thus, we would expect plasmoids to be created more quickly for strong southward IMF than for weakly southward IMF.

On the relationships between double‐onset substorm, pseudobreakup, and IMF variation: The 4 September 1999 event

[1] The relationships between double-onset substorm, pseudobreakup, and IMF variation were investigated with magnetic, auroral, and particle observations from space to the ground during 0200-0600 UT on 4 September 1999. There were five consecutive bursts of Pi2 pulsations on the ground during the time of interest. The onset time of ground Pi2s maps to the same variation sequence in the IMF structure seen propagating to the Earth in multiple satellite observations in the upstream region. The comparison of auroral and energetic particle data with IMF observations shows that a sequence of two double-onset substorms intervened by a pseudobreakup appears in two distinct cycles of southward IMF followed by a northward interval. For the first substorm, the first onset begins when the By magnitude declines after the IMF turns southward for about 90 min, and the second onset occurs after northward turning of the IMF accompanied by an increasing By magnitude. The pseudobreakup appears while the IMF turns southward and the By magnitude slightly decreases. For the second substorm, the first onset commences while the IMF remains southward with a steady By magnitude, and the second onset occurs after the IMF becomes strongly northward and the By magnitude decreases instead. These observations can be explained with the two-neutral-point model. The first onset occurs when the IMF turns southward. Reconnection at the near-Earth neutral point first begins on closed field lines within the plasma sheet, and the second onset occurs when the IMF turns northward and reconnection at the distant neutral point ceases and reconnection at the near-Earth neutral point may reach the open flux of the tail lobes. In addition, a decrease in the By magnitude may help reduce magnetotail convection and release all the built-up flux to allow the onset to commence after northward turning of the IMF. If the IMF remains southward, the reduction of magnetotail convection due to a decreasing By would lead to a pseudobreakup instead. In contrast, an increasing B y magnitude would increase magnetotail convection and weaken magnetospheric substorm after the IMF turns northward. Consequently, for the occurrence of double-onset substorms and pseudobreakups, not only the first onset begins spontaneously during steady southward IMF and the second onset appears after northward turning of the IMF but the By change also affects magnetotail convection which may evoke (or abate) the substorm-related activation while the IMF turns southward (or northward).

Characteristics of consecutive bursts of Pi2 pulsations observed at the SMALL array: A new implication

Consecutive bursts of Pi2 pulsations are examined with magnetic field data obtained by the SMALL array in 1999. With reference to the H-component magnetic bays in the high-latitude magnetograms at CPMN, 10 events consisting of two consecutive Pi2 bursts simultaneously observed by the Beijing (BJI, L = 1.46) and Wuhan (WHN, L = 1.20) stations are identified as associated with substorm onsets. Owing to the same waveform seen by the CPMN and IGPP/LANL arrays, they are the global phenomena. Their occurrences are mostly in the 2100–2300 LT (local time) sector in which the dominant frequencies at WHN are higher than the mean frequency, but those at BJI are lower and close to the frequency of the surface wave at the plasmapause. Moreover, the LT dependence of azimuth and polarization of two consecutive Pi2 bursts at BJI and WHN are analyzed and consistent with the ULF waves theory by Itonaga and Yumoto (1998). GOES 8 and GOES 10 confirm the formation of the substorm current wedge after the onsets of two Pi2 bursts. Thus during substorm onsets, Pi2 pulsations at low latitudes may result from hydromagnetic waves driven by an impulsive source in the magnetotail which could commence in the longitude of 2230 LT and later propagate westward and eastward as well. Low-latitude Pi2 waves near the source site may be affected by several factors as they propagate by the stimulation of a surface wave at the plasmapause, by a localized field line oscillation inside the plasmapause, and by the magnetospheric/plasmaspheric cavity (resonance) mode.

THEMIS observations of consecutive bursts of Pi2 pulsations: The 20 April 2007 event

[1] On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneously in the premidnight sector at the E spacecraft and ground-based observatories for the THEMIS mission, while the AE index was less than 100 nT. Especially for the last three onsets, both ground-based and GOES 12 magnetometers sensed magnetic perturbations, as expected from the formation of a substorm current wedge (SCW). Moreover, LANL 1994-084 detected an enhancement of energetic particle flux. Spectral analysis shows a matched wave frequency ∼6―8 mHz and another harmonic frequency ∼17 mHz for the fourth burst. The orientation of the major axis of the wave polarization hodogram points toward the SCW location. The first burst has both latitudinal and longitudinal polarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to the other three that have a latitudinal reversal only. The longitudinal CW to CCW change at low latitudes signifies that hydromagnetic waves propagate westward and eastward from the longitude of the impulsive source responsible for SCW. The latitudinal CW to CCW reversal is consistent with induction by a westward moving upward field-aligned current carried by Alfven waves leading to field line oscillations. Consequently, they can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances as expected from braking bursty bulk flows, resulting from magnetotail reconnection, triggered by a preceding northward interplanetary magnetic field turning. This event shows that the source mechanism of consecutive Pi2 onsets at times of weak geomagnetic activity is the same as during substorm times.

Association of consecutive Pi2‐Ps6 band pulsations with earthward fast flows in the plasma sheet in response to IMF variations

On 11 March 2009, the H component had four consecutive bay-like variations accompanied by positive and negative deflections in the D component across the Atlantic like those affected by the substorm current wedge formation. A train of pulsations with a frequency range 2–10 mHz (referred to as Pi2-Ps6 band), sensed by Time History of Events and Macroscale Interactions during Substorms (THEMIS)/Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) magnetometers, had clearly three consecutive Pi2s followed by a Ps6 at low latitudes, but first Pi2 and then Ps6 at high latitudes mixed with large-amplitude Ps6 at midlatitudes. The geostationary orbit magnetometers sensed similar magnetic perturbations. THEMIS probes first observed earthward fast flows, magnetic dipolarizations, and modulated energetic particle fluxes at ~ XGSM −9.2 RE, then at ~ XGSM −7.5 RE for Pi2 and at ~ XGSM −18.0 RE only for Ps6. They appeared during a very quiet period for northward interplanetary magnetic field (IMF) with a clock angle variation of low to high and then low. The H spectrum shows two harmonic frequencies ~2–4 mHz and ~8–10 mHz but the D spectrum one dominant frequency ~2–4 mHz. Pi2 can result from a combination of fast magnetospheric and plasmaspheric cavity resonances and Ps6 from a fast magnetospheric cavity resonance. The surface waves at the interface separating braking earthward fast flows from the ambient plasma convection region could lead to large-amplitude Ps6 at midlatitudes. Hence, consecutive Pi2-Ps6 band pulsations can be associated with earthward fast flows in the plasma sheet, expectedly driven by magnetotail reconnection, respectively, in the near-Earth region and the distant Earth one in response to IMF variations as in the two-neutral-point model.