Statistics – Methodology
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsm31a0387c&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SM31A-0387
Statistics
Methodology
2149 Mhd Waves And Turbulence (2752, 6050, 7836), 2744 Magnetotail, 2764 Plasma Sheet, 2790 Substorms, 2794 Instruments And Techniques
Scientific paper
If Pi 2 arrival times observed by multiple satellites and ground stations can be used to infer the location of substorm onset, they could be used to constrain substorm models. The methodology is analogous to locating earthquake hypocenters from the travel time data recorded by an array of seismometers. The Pi 2 travel-time method times the arrival of Pi 2 when the amplitude of its first half cycle reaches maximum, a time corresponding to wave propagation along the ``Tamao travel path,'' which consists of the nearest route from the source to the field line of interest via the fast mode and a segment of field-aligned propagation to the observer via the Alfvén mode. The time that Pi 2 takes to propagate along the Tamao travel path can therefore be estimated by modeling the MHD speeds along the path. We have developed a model of Alfvén and magnetosonic speeds based on the empirical Tsyganenko 1989 magnetosphere model and the observations of plasma density and temperature as those reported by Baumjohann [1993] and by Wing and Newell [1998]. The ratio of the magnetosonic speed to the Alfvén speed can range from 3 to more than 10 near the central plasma sheet due to the high β in the region, and therefore the travel time along the Tamao path is primarily controlled by the Alfvénic portion of the propagation. For substorm onsets that are located farther than X = -12 Re, the Tamao's travel time for Pi 2 propagating from the source to a ground observer varies with the observer's L-value, and it has a local minimum at L ~ 10 and a local maximum at L = 12-20, depending on the source location. The variation in Pi 2 travel time also increases as the onset location moves away from the Earth. Using the data from ground magnetometer arrays in North America as well as those from GOES and Geotail satellites, we analyzed several Pi 2 events and found that their travel time pattern was in good agreement with a location of substorm onset is at approximately at 12-15 Re downtail. Applications of this travel-time method on THEMIS satellite and ground-based data would further illuminate the dynamics of substorms and bursty bulk flows.
Chi Peter J.
Ohtani Shin
Russell Christopher T.
Singer Howard J.
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