Observations of Magnetospheric Substorms Occurring with no Apparent Solar Wind/IMF Trigger
Key Data Sets
LANL Geosynchronous energetic particles,
ISEE1, ISEE3, and IMP8 IMF and solar wind data, worldwide ground magnetometer
data, Viking global auroral imager data
Key Results
Demonstrated that substorms can indeed
occur during steady IMF conditions
Contact
Mike Henderson, Los Alamos National Lab,
505-665-7425, mghenderson@lanl.gov
More Information
Henderson, M. G., G. D. Reeves, R. D. Belian,
and J. S. Murphree, Observations of magnetospheric substorms occurring with
no apparent solar wind/IMF trigger, J. Geophys. Res., 101, 10773-10791,
1996
Henderson, M. G., G. D. Reeves, and R. D. Belian, Multi-point observations
of untriggered substorms, Proc. of the ICS-3, Versailles, France, 12-17
May, pp. 273-278, 1996
Abstract
An outstanding topic in magnetospheric
physics is whether substorms are always externally triggered by disturbances
in either the interplanetary magnetic field (IMF) or solar wind, or whether
they can also occur solely as the result of an internal magnetospheric instability.
Over the past decade, arguments have been made on both sides of this issue.
Horwitz [1985] and McPherron et al. [1986] have shown examples of substorm
onsets which they claimed were not externally triggered. However, as pointed
out by Lyons [1995, 1996], there are several problems associated with these
studies that make their results somewhat inconclusive. In particular, in
the McPherron et al. study, fluctuations in the By component were not considered
as possible triggers. Furthermore, Lyons suggests that the sharp decreases
in the AL index during intervals of steady IMF/solar wind are not substorms
at all but rather that they are just enhancements of the convection driven
DP 2 current system that are often observed to occur during steady magnetospheric
convection events. In the present study, we utilize a much more comprehensive
data set (consisting of particle data from the Los Alamos energetic particle
detectors at geosynchronous orbit, IMP 8 magnetometer and plasma data, Viking
UV auroral imager data, midlatitude Pi 2 pulsation data, ground magnetometer
data, and ISEE 1 magnetic field and energetic particle data) to show as
unambiguously as possible that typical substorms can indeed occur in the
absence of an identifiable trigger in the solar wind/IMF.
The study includes a number of examples one of which is shown below. The
top three panels show LANL energetic particle data that include a classic
dispersionless injection, indicative of substorm onset. The next panel shows
the AL index with a negative "bay" that is also indicative of
substorm onset. The next three panels show IMF data from IMP-8 with dynamic
pressure below that. It is striking that all three components of the magnetic
field as well as the pressure curve are remarkably flat. In particular the
IMF Bz component is steadily southward with no northward turning at or near
the time of substorm onset.. The bottom panels show Kakioka Pi1 and Pi2
data and data from ISEE 1 in the magnetotail. These data sets also show
the classic signature of substorms. This and other examples conclusively
show that substorm can sometimes occur with no solar wind trigger.
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