A study of quasi-periodic ELF-VLF emissions at three Antarctic stations: evidence for off-equatorial generation?

Abstract

Abstract. The spatial extent and temporal behaviour of quasi-periodic (QP) intensity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a comparative study of data collected at the Antarctic stations of South Pole (L=14), Halley (L=4), and Siple (L=4). Frequently, the waveforms of ELF-VLF signals simultaneously received at each site were identical. Although of similar frequency structure, the waveforms of the accompanying Pc3 magnetic pulsations did not show a one-to-one association. Whereas both are dayside phenomena, QP emissions occur over a smaller range of local times, and have a maximum of occurrence later in the day closer to local noon. QP emissions are identified with the periodic modulation of the electron pitch-angle distribution by the propagation of ULF compressional fast-mode waves through a region. However, contrary to previous ideas, rising-tone emissions do not represent the frequency-time signatures of such waves. In addition to generation close to the equatorial plane, we propose an additional high-latitude source of QP emissions. These emissions are associated with regions of minimum B produced by the dayside compression of the magnetosphere close to the magnetopause. Model magnetic field calculations of these minimum-B regions as a function of magnetic local time and invariant latitude are presented.

Latitudinal and seasonal variations of quasiperiodic and periodic VLF emissions in the outer magnetosphere

“…The near‐noon peak in the diurnal profile of QP 1 and QPPE 1 events is consistent with the observations of Morrison et al [1994], again supporting the commonly held view that Pc3‐4 magnetic pulsations are the driver of the observed modulations of ELF/VLF waves (type I QPs). As Smith et al [1998] noted, however, the generation of type II QP events is not well understood.…”

“…This confirmation of the simultaneity of most if not all QP and PE events lends support to the idea advanced by Morrison et al [1994] that each site is seeing a signal from the same relatively localized, ducted whistler event in the magnetosphere. The range of latitudes available in this study thus gives us clues about the L ‐shells on which these events originate.…”

“…In summary, the distribution of all categories of QP events peaked on the day side, as found earlier by Ho [1973] and Morrison et al [1994], while the distribution of PE events without simultaneous QP emissions was strongly latitude‐dependent, with most occurring at subauroral latitudes in a pattern having a minimum in occurrence near local noon.…”

“…A factor of 2–3 more events were observed during summer months at the auroral zone stations, but the latitudinal distribution was roughly the same at all stations during local winter. We believe, as did Morrison et al [1994], that this diurnal pattern reflects the upstream wave source of the Pc3 pulsations that are associated with these QP events, while the seasonal variation in latitude may reflect differences in propagation characteristics: although fewer events occurred during winter, they were usually observed at all four stations. Engebretson et al [2000] also noted, in a comparison between Pc3‐4 observations at South Pole and Sondrestrom (both cusp‐latitude stations) that pulsation power was greater at South Pole during the local winter months.…”

New type of ensemble of quasi-periodic, long-lasting VLF emissions at the auroral zone

“…QP1 is more widely referred to in the literature (e.g., Kimura, 1974;Sato and Fukunishi, 1981;Bespalov and Kleimenova, 1989;Sazhin and Hayakawa, 1994;Manninen et al, 2012). However, there have been relatively few papers analysing QP2 emissions and described their behaviour (Ho, 1973;Morrison et al, 1994;Smith et al, 1998;Engebretson et al, 2004). Based on Antarctic observations, these authors concluded that QP2 are most common in austral summer under conditions of quiet geomagnetic activity, their diurnal maximum of occurrence is near noon, and the period of separated VLF bursts repetition ranged from ∼20 s to ∼100 s. The QP emissions showed good conjugateness and the elements of QP emissions appeared simultaneously at the conjugate stations.…”