Transcript Mission and Validation Overview

Challenges in Measuring External Current Systems Driven
by Solar Wind-Magnetosphere Interaction
Guan Le
NASA Goddard Space Flight Center
Introduction
• In geomagnetism studies, it is always a challenge to
separate magnetic fields from external currents
originating from the ionosphere and magnetosphere.
• The ionospheric and magnetospheric currents are very
dynamic and changes on relatively short and varying
scales.
• The ionospheric and magnetospheric currents are
intimately controlled by the ionospheric electrodynamics
and the solar wind-magnetosphere-ionosphere coupling.
2
External Currents Driven by
Solar Wind-Magnetosphere-Ionosphere Coupling
Magnetospheric
Currents:
• Magnetopause current
• Magnetotail current
• Ring current
MagnetosphereIonosphere coupling:
• Field-aligned Currents
Ionospheric Currents
• Pedersen currents
• Hall currents and
auroral electrojets
(Credit: SwRI)
3
Recent Observations
Space Technology 5 (ST-5)
•Field-aligned currents
•Cross-polar cap Pedersen currents
Communication/Navigation Outage Forecasting
System (C/NOFS)
•The ring current
Space Technology 5
Mission Overview
•
Space Technology 5 (ST-5) is a three microsatellite constellation mission deployed into a
low Earth orbit for technology validation.
•
Mission Duration: 90 days
– Launched March 22, 2006
– Mission completed June 21, 2006
•
Orbit
– Sun-synchronized
– Dawn-dusk meridian plane
– 300 km Perigee
– 4500 km Apogee
– 105.6 deg inclination
– 136 min orbit period
•
Constellation Configuration:
– String of Pearls
– ~ 50 - 5500 km spacing
•
Science Instruments: Miniature fluxgate
magnetometer
Field-Aligned Currents: Separating Space and Time
Temporal Variation
Spatial Variation
6
ST5 Trajectories on April 14, 2006
Southern polar
cap pass near
apogee
Northern polar
cap pass near
perigee
7
ST5 Observations of Field-Aligned Currents (2006-04-14)
UT
Alt_094 (km)
MLT_094 (Hr)
Mlat_094 (deg)
8
Field-Aligned Currents Motion and Temporal Variations (2006-04-14)
Mesoscale ~ 100 km;
Large-scale: ~ 1000 km
9
Ionospheric Closure Currents
Dawn
R2
•
To maintain their divergencefree condition, overall downward
FACs must eventually balance
the overall upward FACs.
•
There is generally an imbalance
in total currents between R1 and
R2 FACs (more currents in R1
than in R2).
R1
R1
Dusk
R2
–
Dawn
Dusk
–
[Le et al., JGR, 2011]
Most of the current closure
takes place via local
Pedersen currents within
auroral zone flowing
between upward and
downward FACs;
The net currents due to the
R1-R2 imbalance can be
closed within R1 via crosspolar cap Pedersen
currents.
10
Closure of Field-aligned Currents in the Ionosphere
ST-5 Observations
[Le et al., JGR, 2011]
11
Summary of ST5 Observations
• ST5’s multi-point data can be used to separate FACs’ temporal and spatial
variations on time scales of ~ 7 – 700 s and spatial scales of ~ 50 - 5500 km.
• During active periods, meso-scale current structures are very dynamic.
- Highly variable in current density and/or polarity at ~ 10 min time scale
- Relatively stable at ~ 1 min time scale
• Large-scale currents are relatively stable at ~ 10 min time scales.
• ST5 observations show that Region 1 currents are generally stronger than
Region 2 currents in both the dawnside and duskside. The imbalance between
R1 and R2 currents indicates that:
- ~ 95% of FACs are closed by local Pedersen currents flowing equatorward in the
dawnside (poleward in the duskside) within the auroral zone between the upward and
downward FACs.
- ~ 5% of FACs are closed via cross-polar cap Pedersen currents flowing from dawn to
dusk. The total amount of the cross-polar cap Pedersen currents is in the order of ~
0.1 MA.
ST5 data are available publicly at the Virtual Magnetospheric Observatory (VMO) at
http://vmo.gsfc.nasa.gov/.
12
Communication/Navigation Outage Forecasting System
C/NOFS Mission Overview
•
C/NOFS was launched into a low
inclination, elliptical orbit on April
16, 2008.
Inclination: 13 degrees
Orbit Perigee: 401 km
Orbit Apogee: 867 km
Orbit period: 97 min
•
C/NOFS provides a complete
coverage of all local times
every 97 minutes, which allows
us to study the temporal and
local time variations of the ring
current during geomagnetic
storms.
13
Magnetic Field Observations
Local Time Variations of Northward Magnetic Residuals
•
C/NOFS magnetic field data are first
calibrated using the latest POMME model.
dBN = BN(C/NOFS) – BN (IGRF)
•
Presentation of the Data
- Dashed circle: Baseline dBN=0
- Blue circle: Dst index
- Thick black trace:
dBN data versus the local time
- Red circle:
least square fit of the data
• Center of the fitting circle
• Radius of the fitting circle
14
The May 29, 2010 Magnetic Storm
15
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (1/8)
16
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (2/8)
17
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (3/8)
18
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (4/8)
19
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (5/8)
20
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (6/8)
21
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (7/8)
22
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm (8/8)
23
Magnetic Field Observations
During the May 29, 2010 Magnetic Storm
24
Provisional Dst Index
Estimated from C/NOFS MAG Data
The May 29, 2010 Storm
25
Summary of C/NOFS Observations
• C/NOFS measurements consistently show that the magnetic
field residuals (and the ring current) is very asymmetric in local
time during the main phase of geomagnetic storms. The ring
current becomes less asymmetric during the recovery phase,
but remains asymmetric in local time.
• It is feasible to measure provisional Dst index in real time using
magnetic field data from low altitude equatorial satellites.
26
Conclusions
Single spacecraft observations are not able to separate spatial
and temporal variations, and thus to accurately describe the
configuration of the external current system. A constellation
mission with a combination of low and high inclination spacecraft
is needed.
•A single low inclination spacecraft is able to monitor the
temporal evolution and local time distribution of the ring current
during magnetic storms. It can also provide near real time
provisional Dst index.
•Multiple high inclination spacecraft with different orbital
configurations are needed to specify high latitude currents.
-Two or more spacecraft in the same orbit (string-of-pearl)
-Multiple local times
-Simultaneous monitoring in the northern and southern polar
regions
27
Backup Charts
28
ST5 Mission and Orbit Profile
Mission Duration
90 days
(March 22 – June 21)
Orbit
Sun-synchronized
Dawn-dusk meridian
plane
Inclination
105.6º
Orbit Perigee
300 km
Apogee
4500 km
Period
136 minute
Constellation
configuration
String-of-Pearls
~ 50- ~ 5000
spacing
224
km
094
155
29