Transcript Identification and Analysis of Magnetic Substorms Patricia

Identification and Analysis
of Magnetic Substorms
Patricia Gavin1, Sandra Brogl1, Ramon Lopez2, Hamid Rassoul1
1.
Florida Institute of Technology, Physics and Space Science Department, 2. University of Texas at Arlington, Department of Physics
Abstract:
Using AE data from [1], we have identified 218 isolated substorms whose initial onset is over North America (between 0300UT and 0800UT). We constructed a data table that
contains each substorms’ onset time, strength in AE, duration, and whether or not the event was a multiple- or single-onset substorm. We have examined the statistics of these
events, in particular comparing single-onset and multiple-onset substorms. Preliminary results indicate that the strengths of both single- and multiple-onset substorms are very
similar. The investigations done here determined the weak relationships between aspects of the substorms, such as substorm’s maximum strength and duration. We have
collected data about the substorms from satellites orbiting Earth and hope to put this data into a computer model to help further understand these events.
Results (cont’d)
Tools and Methods:
Substorms are an important tool in understanding the interactions between Earth’s magnetic field and
solar wind. A substorm is a magnetic disturbance lasting only a few hours in Earth’s magnetotail. The
life of a substorm consists of three phases: the growth phase, the expansion phase and the recovery
phase.
The intensity of a substorm is measured in the Auroral Electrojet (AE) index.
AE is simply the difference between the upper auroral (AU) index and the
lower auroral (AL) index. It is a normalized value derived from the Hcomponent from several observatories along the auroral zone. It measures the
amount of energy being inputted into the ionosphere from Earth’s magnetotail
[4],[5]. Substorms are divided into two categories: single-onset and multipleonset.
Figures 3 and 4 show examples of single- and multiple-onset
substorms, respectively. They are identified by a sharp increase in AE and a
gradual recovery afterward.
During the growth phase, when the Bz (vertical) component of the interplanetary magnetic field (IMF),
carried by the plasma of the incoming solar wind, points southward, it connects with Earth’s northward
pointing magnetic field lines on Earth’s day side, creating open field lines. An open field line in this
case would indicate a magnetic field line whose one end is connected to the IMF and other end is
connected to the Earth’s magnetic field. On Earth’s night side, however, the IMF lines reconnect,
creating closed field lines (a loop of magnetic field lines) that elongate radially outward from the sun
(Figure 1) storing energy in the magnetotail. Both ends of these lines are connected to Earth’s poles
where they deposit charged plasma particles into Earth’s ionosphere [2].
Figure 3:
February 7th, 1998
Onset: 0513 UT
Strength: 179 nT
Duration: 0.48 hrs
The arrow indicates the
onset of the substorm
[1].
Figure 4:
February
11th, 1999
1st Onset: 0537
UT
1st Onset Strength:
496 nT Max AE: 622 nT
Duration: 2.65 hrs
During the recovery phase, the substorm
current wedge stops and the magnetic field
and auroral bulge gradually return to their
original state [3].
Figure 2: The release of a plasmoid down the Earth’s magnetotail. (1) shows where the energy is stored. (2)
shows the release of the plasmoid. (3) shows the magnetotail returning to normal.
Figure 6: A comparison of the
duration of both types of
substorms. The average duration
of a multiple-onset substorm was
about 0.5 hours longer than that
of a single-onset substorm.
Duration Distribution
Single-Onset
Multiple-Onset
70
60
50
40
30
20
10
0
0-0.99
1.00-1.99
2.00-2.99
3.00-3.99
4.00-4.99
Duration (hrs)
Multiple-onset Substorms:
It was determined that for most of the multiple-onset substorms investigated, the first onset was the
strongest. Most multiple-onset substorms reached their peak AE between 200 and 599 nT. There
seemed to be a weak correlation between the number of onsets in a multiple-onset substorm and its
maximum AE. There was also a weak relationship between the multiple-onset substorms’ maximum
AE and their duration.
The final goal of this project is to collect data about substorms and plug it into a computer model.
This will help scientists further understand the physics behind substorms and possibly be able to
predict these phenomena. Currently we are collecting data from satellites orbiting the Earth that
take data on things like the strength and direction of the magnetic field and the solar wind
velocity. We are looking for pairs of satellites, one in the magnetotail and one in the plasmasheet
(See Figure 1). Figure 7 shows an example of such an orbital orientation from April 30th to May
4th, 2000. In this case, we will take data from Wind (black) and Imp-8 (blue) on day 124.
Results and Conclusions:
Strength Distribution:
Figure 5 shows a comparison of the strength of the single-onset substorms that
were investigated and the strength of the first onset of the multiple-onset
substorms that were investigated. Both plots peak in the 300-399 nT range
indicating a similarity of the two.
Strength Distribution
Single-Onset
Multiple-Onset
30
25
Quantity
During the expansion phase, the onset of the
substorm current wedge occurs. When the
reconnection of the IMF lines occurs rapidly,
the closed field line region snaps back into
place, releasing a plasmoid (loop of closed
field lines) down the magnetotail (Figure 2).
The resulting release of high-energy plasma
particles into the ionosphere is called a
substorm. These incoming energetic particles
increase the intensity of and expand the
auroral arc on Earth’s night side.
Figure 6 shows a comparison of the duration of the two types of substorms. The average duration of the
single-onset substorms was 1.36 hours and the average of the multiple-onset substorms was 1.90 hours.
Thus, on average, multiple-onset substorms lasted longer than single-onset substorms by about 0.5
hours.
Current and Future Work:
The arrows indicate the first
and second onsets (five total)
[1].
Figure 1: A schematic of the Earth’s magnetic field
Duration Distribution:
Quantity
Introduction:
Figure 7: An example of the orientation of satellites from which we will take data to plug into a computer
model of substorms. Earth lies at (0,0) in both plots. The z-axis goes through the Earth’s poles and
Earth’s equator lies in the x-y plane.
20
15
10
References:
5
0
0-99
100199
200299
300399
400499
500599
600699
700799
800899
900999
10001099
11001199
12001299
13001399
14001499
15001599
16001699
Strength (nT)
Figure 5: A comparison of the strength distribution of single-onset
substorms and multiple-onset substorms. Both plots peak in the 300-399
nT range indicating a similarity between the two types of substoms.
[1] http://swdcwww.kugi.kyoto-u.ac.jp/ae_realtime/index.html, December 2007. [2] Baker, D. N. (1996). Magnetic
Reconnection During Magnetospheric Substorms, NASA Astrophysics Data System, 365 – 372. [3] Lopez, R. E. (1990).
Magnetospheric Substorms, Johns Hopkins APL Technical Digest 11, 264 – 271. [4] Kisabeth, J. L., and Rostoker, G.
(1974). The Expansive Phase of Magnetospheric Substorms – Development of the Auroral Electrojects and Auroral Arc
Configuration During a Substorm, JGR 79, 972 – 984. [5] Lopez, R. E., and von Rosenvinge, T. (1993). A Statistical
Relationship Between the Geosynchronous Magnetic Field and Substorm Electrojet Magnitude, JGR 98, 3851 – 3857.