1. Geomagnetic Activity: A Talk on “Current Events”!
Christine Gabrielse, UCLA
GEM Tutorial, June 17, 2012
“Activity” implies there is an active time and a quiet time.
Changes in currents above the ionosphere controlled by solar wind parameters  changes in the magnetic field at earth’s surface.

2. Storms
Symmetric Ring Current
Dipole Axis
Magnetic Field Lines
Sudden Commencement 50
Dst Index:
Direct measure of the hourly average of the northward (“H”) perturbation in the magnetic field at equatorial and mid-latitudes
Measures symmetric ring current strength
Main Phase
The coronal mass ejections from the Sun, with velocities up to 2000 km/s, give raise to shock-waves in the solar wind. The related pressure pulses, when impinging the Earth's magnetosphere, both compress it and increase the magnetopause current. This leads into a few tens of nT intensifications in the low-latitude ground-based magnetic field intensity, lasting typically for some tens of minutes. These signatures are called Sudden Storm Commencements (SSC) or Sudden Impulses (SI), depending whether a magnetospheric storm is initiated or not.
Storms, the main contributors to space weather, are initiated when enhanced energy transfer from the solar wind/IMF into the magnetosphere leads into intensification of ring current. The ring current development can be monitored with the Dst index.
DST index: At such latitudes the H (northward) component of the magnetic perturbation is dominated by the intensity of the magnetospheric ring current. Dst index is a direct measure of the hourly average of this perturbation.
The increased dayside reconnection increases the penetration of the solar wind into the magnetosphere.
The enhanced duskward electric field increases the number of particles injected into the ring current.
Stronger electric fields lead to earthward expansion of the ring current region.
Ampere’s Law
Operational purposes: Dst < -50 for storm
Symmetric ring current forms during storms: different theories. Increased convection, substorm expansion phase, injections.
Auroral oval moves southward. Stronger convection (E=-vxB) makes Alfven layer shrink,
SW conditions: Bz < -10 nT for >3 hours
Dst
Recovery
Phase
-100
-150
134
136
Day
142
144

3. Substorms timescale of ~1-3 hours Doesn’t need CME.
Period of long southward Bz, Northward turning of Bz suggested trigger.

4. Auroral Electrojet H Z Substorm Current Wedge
Figure 2. IMAGE Magnetometer chain. Figure 2 (a)-(c) demonstrate the x-, z-, and y-component of the magnetic field observed by six stations in Norway during the time surrounding the 18:12 substorm. The black vertical line in (a) and (c) marks the onset of the previous substorm simultaneously observed at all stations, implying the signature is more global, and the substorm is located further away. The red vertical lines mark the onset of the event we study at each individual station. Evidenced from the delayed onset times, we find the current propagates poleward from BJO to just north of HOR. (b) demonstrates this fact as the z-negative bay implies a current system to the north while a z-positive bay implies the system is to the south. The red horizontal line marks zero. (d) illustrates the locations of the different stations (the six northernmost).
Substorm Current Wedge

5. Substorm Indices
Kp Index: Perhaps best to monitor substorm expansion phase during storms. 0-4 is below storm K nT
0-5 1
5-10 2
10-20 3
20-40 4
40-70 5
70-120 6 7 8 9
>500
These disturbances in the H-component of the magnetic field are recorded into a series of indices.
A superposition of these data from all the stations enables a lower bound or maximum negative excursion of the H component to be determined; this is called the AL index. Similarly, an upper bound or maximum positive excursion in H is determined; this is called the AU index. The difference between these two indices, AU-AL, is called the AE index. Notice that negative H perturbations occur when stations are under an westward-flowing current. Thus the indices AU and AL give some measure of the individual strengths of eastward and westward electrojets, while AE provides a measure of the overall horizontal current strength.
Kp index is a mid-latitudes. Measure of field-aligned currents flowing into and out of the auroral oval, primarily associated with SCW.
Site dependent.
Log scale.
Weighted average of mid-lat stations
AU: Amplitude Upper
AL: Amplitude Lower
AE: Auroral Electrojet = AU - AL
AO: (AU + AL) / 2

6. Substorm Signatures Runov et al., 2011
Geosynchronous spacecraft observe repeatable signatures during the substorm cycle. As magnetospheric magnetic field lines stretch during the substorm growth phase, spacecraft on the nightside may leave the radiation belts and enter the magnetotail lobes. Particle fluxes diminish over a period ranging from 30 min to 1 hour.
At substorm onset, the magnetic field lines collapse into a dipolar configuration and spacecraft on the nightside return to the particle rich radiation belts within a period of minutes. The clouds of injected particles drift around the Earth to form the radiation belts. After a sufficient time, which depends upon particle energy, the injected particles can be observed drifting through the dayside magnetosphere.
The LANL plot for energetic electrons observed by spacecraft on March 10, 1998 presents a clear growth phase signature from 0600 UT, ending in an onset at 0900 UT.
In these plots different panels correspond to different spacecraft. The different curves represent differential fluxes in different energy bands. The exact energies measured by the different spacecraft vary, but are within the keV range.

7. Sawtooth Events: Identity Crisis
Substorms?
Solar wind driven?
Similar signatures
Driving conditions are stronger than those during substorms and SMCs
Similar conditions as CME: Storm time event?
3 hour periodicity > Magnetosphere cavity oscillations
Not substorms?
Signatures occur over wide range in MLT
Periodic
More intense at geostationary orbit
Ion outflows may play a role deciding if we get SMC or sawtooth event [Brambles et al., 2011]
occur during storms when the ring current is enhanced and are driven by moderate (Bz ] 10 nT) and steadily southward IMF conditions. The oscillations have been termed ‘sawtooth events’ because their shape – a series of slow flux decreases followed by rapid increases – resemble the teeth of a saw blade. The ‘sawtooth’ shape is particularly well-defined in the high energy proton channels
Dynamic pressure increases occur at all MLTs
Repetivie ss under strong driving?
Large substorms driven internally by Msph with periodicity of 3 hours
Is it really periodic? How is it driven internally? Are they
Still a dychotomy. Only studied with LANL (geosynch.)
Internally driven?
Occur during stable IMF
Similar conditions as steady magnetospheric convection 7

8. Steady Magnetospheric Convection
Kissinger et al. , 2011, 2012
Timescale: >4-6 hours
Many are initiated by substorm
Balanced dayside/nightside reconnection
Auroral zone currents dominated by convection-driven current system, not substorm related currents
Midlatitude disturbances from partial ring current
Substorm
SMC
-20
-20
Y GSM 20 20
-35
X GSM
-35
X GSM

9. Relevant GEM Focus Groups
Modes of Magnetospheric Response
SMCs, drivers/effects of other convection modes: sawtooth events, non-linear coupling to the solar wind driver, polar cap saturation, ion outflow, ionospheric effects, and global simulation results
Monday: 10:30, 1:30, Salon D
Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures
Magnetic reconnection, FTEs, and Hot Flow Anomalies
Tuesday: 1:30, 3:30, Salon C
Substorm Expansion Onset: The First 10 Minutes
Signature propagation and timing from tail toward the inner magnetosphere and to the ground; signatures measured in the distant tail
Tuesday: 10:30, 1:30, 3:30, Salon D
Dayside FACs and Energy Deposition
Relation between enhanced dayside Poynting flux and field-aligned currents, sources of field-aligned currents in the solar wind and magnetosphere and their impacts in the ionosphere-thermosphere system
Thursday: 10:30, Salon C

10. References Slide 1 Slide 2 Slide 3 Slide 4
Image courtesy of USGS:
SOHO movie of CME courtesy of NASA/GSFC
SDO movie of prominence erupting courtesy of NASA
Slide 2
CME movie courtesy of the Center for Space Environment Modeling (CSEM), University of Michigan.
Figures courtesy of R. McPherron’s UCLA class notes
Slide 3
Substorm movie courtesy of NASA:
Figure: Miyashita et al. (2009), J. Geophys. Res., doi: /2008JA013225
Slide 4
Substorm current wedge figure courtesy of R. McPherron class notes
IMAGE plots created by J. Reistad at UNIS Substorm class, 2010

11. References Slide 5 Slide 6
AE index courtesy of WDC Kyoto: wdc.kugi.kyoto-u.ac.jp/ae_provisional/
Kp index:
Rostoker, Ann. Geophysicae 18, (2000)
FAC image: Keiling et al., Substorm current wedge driven by plasma flow vortices: THEMIS observations, J. Geophys. Res., 114, A00C22, doi: /2009JA
Slide 6
Substorm signatures figure: Runov, A., V. Angelopoulos, X.‐Z. Zhou, X.‐J. Zhang, S. Li, F. Plaschke, and J. Bonnell (2011), A THEMIS multicase study of dipolarization fronts in the magnetotail plasma sheet, J. Geophys. Res., 116, A05216, doi: /2010JA
Substorm injection figure courtesy of LANL:
Substorm figure modified from Miyashita et al. (2009), J. Geophys. Res., doi: /2008JA
Injection boundary figure: Mauk and Meng, JGR, VOL. 86, NO. A4, PAGES , APRIL 1, 1983

12. References Slide 7 Slide 8 SMC figures:
Sawtooth injection figure: Kubyshkina, M., T. I. Pulkkinen, N. Yu. Ganushkina, and N. Partamies (2008), Magnetospheric currents during sawtooth events: Event-oriented magnetic field model analysis, J. Geophys. Res., 113, A08211, doi: /2007JA012983
Sawtooth info
Cai, X., J.‐C. Zhang, C. R. Clauer, and M. W. Liemohn (2011), Relationship between sawtooth events and magnetic storms, J. Geophys. Res., 116, A07208, doi: /2010JA
Henderson, M. G. (2004), The May 2–3, 1986 CDAW-9C interval: A sawtooth event, Geophys. Res. Lett., 31, L11804, doi: /2004GL
Huang, C.-S., A. D. DeJong, and X. Cai (2009), Magnetic flux in the magnetotail and polar cap during sawteeth, isolated substorms, and steady magnetospheric convection events, J. Geophys. Res., 114, A07202, doi: /2009JA
Brambles et al., Science 332, 1183 (2011); DOI: /science
Slide 8
SMC figures:
Kissinger, J., R. L. McPherron, T.-S. Hsu, and V. Angelopoulos (2011), Steady magnetospheric convection and stream interfaces: Relationship over a solar cycle, J. Geophys. Res., 116, A00I19, doi: /2010JA
Kissinger, J., R. L. McPherron, T.-S. Hsu, and V. Angelopoulos (2012), Diversion of plasma due to high pressure in the inner magnetosphere during steady magnetospheric convection, J. Geophys. Res., 117, A05206, doi: /2012JA

13. Extra

14. Schematic Illustration of Effects of Ring Current in H Component
Projection of a uniform axial field onto Earth’s surface
The coordinate system used in magnetic measurements of a magnetic storm
The left panel displays how this magnetic field is measured on the Earth at the magnetic latitude of Los Angeles.
The axial field at a station Bi is projected onto the local tangent plane. We measure the Hi component and convert it to the equivalent axial field Bi.
Note the cos(maglat) in the denominator.
This correction only applies to the ring current. Other current systems produce magnetic perturbations that do not necessarily obey this rule.
The right panel shows the details of the coordinate system used to measure magnetic fields. X, Y, Z is a local geographic Cartesian system with X north, Y east, and Z down. H is the projection of the instantaneous magnetic field vector F into the local horizontal plane. D is the angle the projection H makes with the X (geographic north) axis. The angle I is the inclination of the field F below (north hemisphere) the tangent plane.

15. Sawtooth Events
Series of particle injection events [Belian et al., 1995]
Occur over a wider local time extent (than SS)
Injection boundary can include the entire nightside extending past the terminators
Quasiperiodic oscillations: min with large standard deviation of 54.0 min [Cai and Clauer, 2009]
Triggered by solar wind discontinuities (e.g., pressure jumps [Lee et al., 2004] and/or interplanetary magnetic field (IMF) changes [Henderson et al., 2006a]
Related to an intrinsic period of the Earth’s magnetosphere and is independent of external solar wind drivers [Huang et al., 2003a; Henderson et al., 2006a]
Periodic substorms, or something altogether different?
Similar, except the particle injection boundary and the near‐Earth reconnection site are closer to the Earth than that during typical substorms [Henderson, 2004]
Special phenomena associated with unique solar wind conditions
Alfvénic Mach number is low
Particle injection is globally dispersionless, even around the local noon [Borovsky et al., 2009]
Solar wind driving conditions during sawtooth events are stronger than those during typical substorms and SMCs [DeJong et al., 2008])
The statistical characteristics of the solar wind agree with those during an interplanetary coronal mass ejection (ICME)
Since ICMEs are capable of driving magnetic storms [e.g., Gonzalez et al., 1999], there is a hypothesis that sawtooth events are storm time phenomena
Occur during storms when ring current is enhanced
Driven by moderate (Bz~10 nT) and steadily southward IMF conditions
driven by extremely strong solar wind and continuously southward IMF during magnetic storms, and the magnetospheric-ionospheric disturbances during sawtooth events are generally much larger than those during quiet time isolated substorms
occur during storms when the ring current is enhanced and are driven by moderate (Bz ] 10 nT) and steadily southward IMF conditions. The oscillations have been termed ‘sawtooth events’ because their shape – a series of slow flux decreases followed by rapid increases – resemble the teeth of a saw blade. The ‘sawtooth’ shape is particularly well-defined in the high energy proton channels
Dynamic pressure increases occur at all MLTs
Repetivie ss under strong driving?
Large substorms driven internally by Msph with periodicity of 3 hours
Is it really periodic? How is it driven internally? Are they
Still a dychotomy. Only studied with LANL (geosynch.)

16. (1) are sawtooth events storm time phenomena,
(2) is there a sawtooth interval during each storm regardless of storm strength,
(3) does the sawtooth occurrence rate depend on the strength of the ring current, and
(4) is there a preferred storm phase for sawtooth occurrence?

17. Sawtooth Events
The Debate: Quasi-periodic (~3 hour) substorms, or something altogether different? Are solar wind fluctuations the driver, or is an internal (magneospheric) source?
Similar signatures: injections, dipolarizations, AE indices, auroral precipitation
But periodic and widespread (affecting large range of MLT).
Similar, except the particle injection boundary and the near‐Earth reconnection site are closer to the Earth than that during typical substorms [Henderson, 2004]
Special phenomena associated with unique solar wind conditions
Particle injection is globally dispersionless, even around the local noon [Borovsky et al., 2009]
Dynamic pressure increases at all MLTs
Solar wind driving conditions during sawtooth events are stronger than those during typical substorms and SMCs
The statistical characteristics of the solar wind agree with those during an interplanetary coronal mass ejection (ICME)
Since ICMEs are capable of driving magnetic storms, there is a hypothesis that sawtooth events are storm time phenomena
Occur during storms when ring current is enhanced
Driven by moderate (Bz~10 nT) and steadily southward IMF conditions
driven by extremely strong solar wind and continuously southward IMF during magnetic storms, and the magnetospheric-ionospheric disturbances during sawtooth events are generally much larger than those during quiet time isolated substorms
occur during storms when the ring current is enhanced and are driven by moderate (Bz ] 10 nT) and steadily southward IMF conditions. The oscillations have been termed ‘sawtooth events’ because their shape – a series of slow flux decreases followed by rapid increases – resemble the teeth of a saw blade. The ‘sawtooth’ shape is particularly well-defined in the high energy proton channels
Dynamic pressure increases occur at all MLTs
Repetivie ss under strong driving?
Large substorms driven internally by Msph with periodicity of 3 hours
Is it really periodic? How is it driven internally? Are they
Still a dychotomy. Only studied with LANL (geosynch.) 17