09:31 〜 09:53
[PEM15-03] Solar Radar
★招待講演
キーワード：space weather, radar, solar corona
The prospects of probing the solar corona, solar prominences, and
coronal mass ejections (CMEs) from the ground using a large radar will
be examined. Solar radar would utilize direct reflection
(i.e. soundings) from the solar plasma supplemented by coherent
scatter from Langmuir waves in coronal arcs and CMEs. Active sounding
could provide unambiguous information about the range, bearing, and
speed of the targets. Such information would be crucial for
initial-value and assimilative space-weather models providing
operational space-weather forecasts.
Challenges posed by solar-radar are significant but manageable, and
many of the design choices are clearcut. For solar studies, the radar
wavelength must be longer than the plasma Debye length. This places a
premium on low radar frequencies which overrides the penalty of
increased sky and solar noise. However, the radar frequency should not
fall below the maximum usable frequency (MUF) since that would invite
radar clutter from sky waves. The ideal frequency is therefore between
40--50 MHz. The most important parameter is the transmitter
power-aperture product which limits the flux that can be delivered to
the Sun. To optimize this flux, the antenna for transmission should
be a steerable aperture or filled array with about a 1-degree
half-power beamwidth. Steerability is required to keep the radar beam
trained on the Sun, facilitating long incoherent integration
times. The receive array meanwhile must be large enough that most of
the noise it receives comes from the solar disk itself and not from
the galactic background. However, we must consider that the main
source of noise will be type III radio bursts. The noise temperature
at VHF frequencies from solar radio bursts can be several orders of
magnitude greater than that of the quiet sun, and system performance
will depend on discriminating solar echoes from radio bursts. Adaptive
beamforming will ultimately be critical for operational solar-radar
space-weather applications. It is in this way that a large, modular
receiving arrays become important.
All things considered, a facility comparable in size and power to the
existing NSF Geospace Facilities but operating in the VHF band and
possessing spaced-receiver capabilities should be able to detect solar
echoes. Several attempts have been made already to detect solar
echoes. The historical record is mixed, and the plausibility of the
concept remains somewhat ambiguous. Recent and ongoing attempts to
receive solar echoes at The Jicamarca Radio Observatory near Lima,
Peru, will be discussed.
coronal mass ejections (CMEs) from the ground using a large radar will
be examined. Solar radar would utilize direct reflection
(i.e. soundings) from the solar plasma supplemented by coherent
scatter from Langmuir waves in coronal arcs and CMEs. Active sounding
could provide unambiguous information about the range, bearing, and
speed of the targets. Such information would be crucial for
initial-value and assimilative space-weather models providing
operational space-weather forecasts.
Challenges posed by solar-radar are significant but manageable, and
many of the design choices are clearcut. For solar studies, the radar
wavelength must be longer than the plasma Debye length. This places a
premium on low radar frequencies which overrides the penalty of
increased sky and solar noise. However, the radar frequency should not
fall below the maximum usable frequency (MUF) since that would invite
radar clutter from sky waves. The ideal frequency is therefore between
40--50 MHz. The most important parameter is the transmitter
power-aperture product which limits the flux that can be delivered to
the Sun. To optimize this flux, the antenna for transmission should
be a steerable aperture or filled array with about a 1-degree
half-power beamwidth. Steerability is required to keep the radar beam
trained on the Sun, facilitating long incoherent integration
times. The receive array meanwhile must be large enough that most of
the noise it receives comes from the solar disk itself and not from
the galactic background. However, we must consider that the main
source of noise will be type III radio bursts. The noise temperature
at VHF frequencies from solar radio bursts can be several orders of
magnitude greater than that of the quiet sun, and system performance
will depend on discriminating solar echoes from radio bursts. Adaptive
beamforming will ultimately be critical for operational solar-radar
space-weather applications. It is in this way that a large, modular
receiving arrays become important.
All things considered, a facility comparable in size and power to the
existing NSF Geospace Facilities but operating in the VHF band and
possessing spaced-receiver capabilities should be able to detect solar
echoes. Several attempts have been made already to detect solar
echoes. The historical record is mixed, and the plausibility of the
concept remains somewhat ambiguous. Recent and ongoing attempts to
receive solar echoes at The Jicamarca Radio Observatory near Lima,
Peru, will be discussed.