Институт физики Земли им. О.Ю. Шмидта РАН
Геофизический Центр РАН
Москва, Россия
Геофизический центр РАН
Москва, Россия
Центр космической науки в Биркеланне
Свалбард, Норвегия
Центр космической науки в Биркеланне
Свалбард, Норвегия
Апатиты, Россия
УДК 52 Астрономия. Астрофизика. Исследование космического пространства. Геодезия
В геофизической литературе активно обсуждается гипотеза о том, что поверхностные волны на магнитопаузе могли бы быть источником дневных широкополосных пульсаций диапазона Pc5–6 (~1–2 мГц) на околокасповых широтах. Однако первые попытки найти наземный отклик на эти колебания не дали обнадеживающих результатов. Так, сопоставление дневной границы открытых и замкнутых силовых линий (т. н. OCB — Open-Closed field line Boundary) по данным радара SuperDARN с пространственной структурой Pc5–6 пульсаций, зарегистрированных на сети магнитометров на околокасповых широтах, показало, что спектральная мощность этих пульсаций имеет максимум преимущественно на 2–3° более внутренних оболочках магнитосферы, чем положение OCB. Дополнительно связь между OCB и геомагнитными пульсациями исследована путем сопоставления профиля мощности дневных пульсаций с экваториальной границей аврорального овала, определяемой по красной кислородной эмиссии по данным сканирующего фотометра на Свалбарде. В большинстве проанализированных событий «эпицентр» мощности Pc5–6 пульсаций оказался примерно на 1–2° более низкой широте, чем OCB, определенной по оптическим данным. Таким образом, дневные пульсации диапазона Pc5–6 нельзя напрямую связывать с поверхностными модами на магнитопаузе или колебаниями последней замкнутой силовой линии. Отсутствие наземного отклика на эти моды в области ионосферной проекции OCB кажется удивительным. В качестве возможного объяснения мы предположили, что сильная вариабельность внешней магнитосферы вблизи магнитопаузы может подавлять возбуждение колебаний в этой области. Для количественной проверки этой гипотезы, нами рассмотрено возбуждение внешним источником МГД-резонатора между сопря-женными ионосферами со стохастическими флуктуациями собственной частоты. Решение этой задачи действительно показывает существенное ухудшение резонансных свойств МГД-резонатора даже при сравнительно низком уровне фоновых флуктуаций. Этот эффект в принципе может объяснить, почему наземный отклик на магнитосферные колебания отсутствует в области проекции OCB, но наблюдается на более внутренних магнитных оболочках, где структура магнитного поля и плазмы более стабильна.
УНЧ-волны, магнитопауза, граница замкнутых и разомкнутых силовых линий, поверхностные МГД-моды, альвеновский резонатор
IONOSPHERIC FOOTPRINT OF THE DAYSIDE OCB FROM OPTICAL AND HF RADAR OBSERVATIONS
The open-closed field line boundary (OCB) is the boundary that separates magnetospheric field lines that are dragged by the interplanetary magnetic field from those that are closed within the magnetosphere. The ability to monitor the OCB location allows us to study the electrodynamics of magnetosphere–ionosphere coupling, in particular to estimate energy storage and release in the magnetosphere. There are several methods for discriminating the ionospheric footprint of the dayside OCB, but each of them has some merits and drawbacks.
The open/closed separatrix can be identified using particle precipitation boundaries observed by low-altitude spacecraft [Newell, Meng, 1988; Oksavik et al., 2000]. The poleward edge of high-energy (>10 keV) electron precipitation (‘‘trapping boundary’’) corresponds to particles trapped on closed field lines. Field lines poleward of this region must therefore be open. However, the measurements can be made with cadence about 90 min and along orbit only.
Precipitating soft electrons produce plasma irregularities, and strong coherent backscatter from these irregularities can be monitored by Super Dual Auroral Radar Network (SuperDARN) radars. The spectral width of the Doppler velocity of measured backscattered signal can be interpreted as a manifestation of plasma turbulence of the scattering region. The transition from narrow to broad spectral widths has been utilized to investigate the location of ionospheric boundaries [Baker et al., 1995]. Many studies have revealed that the high-latitude broad Doppler spectral width can be used as an indicator for the cusp region [Moen et al., 2001], even though the interpretation of the broad spectral width is still elusive. However, ionospheric regions with elevated turbulence level can be associated not only with the cusp proper, but with the low-latitude boundary layer; hence, this method may provide ambiguous results.
A demarcation of the cusp soft electron precipitation can be determined by comparing the equatorward boundary of the red (630.0 nm) auroral emission along the photometer meridian scanning [Lorentzen et al., 1996]. Precipitation of soft electrons with 0.1–1 keV energies is a good indicator of open magnetic field lines and corresponds to the cusp aurora dominated by the red emission (Rayleigh intensity ratio 630.0/557.7>1) [Lorentzen, Moen, 2000; Johnsen, Lorentzen, 2012]. However, dayside auroral optical measurements can be made during winter months only.
The co-location of the equatorward edge of the HF radar cusp and the cusp auroral emission has been extensively studied [Yeoman et al., 1997]. Rodger et al. [1995] showed that the radar cusp signatures were located ~0.5° equatorward of the optical cusp, whereas Milan et al. [1999] found that the equatorward boundaries of the optical cusp were, on average, located at slightly lower latitudes than the radar cusp. Comparing the spectral width method to the optical method, the radar OCB proxy was seen to be on average 0.5–1.6° (56–170 km) poleward of the optical OCB [Chen et al., 2015].
A new discriminator of OCB was suggested on the basis of ground magnetometer observations of high-latitude ultra-low-frequency (ULF) activity. The background of this tool is described in the following section.
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