Substorm studies with magnetic field measurements from ground-based networks

Transcription

1 Substorm studies with magnetic field measurements from ground-based networks Eija I. Tanskanen University of Bergen,Institute of Physics and Technology, Bergen, Norway Finnish Meteorological Institute, Space Research Unit, Helsinki, Finland ICESTAR Kick-off meeting, Helsinki FMI/AVA

2 Outline Existing ground-based magnetometer networks Critical data gaps -- > need for new observatories? A tool for substorm research: an algorithmical method for substorm identification Substorm research with magnetometers: summary of results Future ICESTAR-related projects

3 Northern hemisphere aurora region arrays Kyoto observatories, 12 magnetometers CARISMA (earlier: Canopus) MACCS Greenland array IMAGE 210 chain Themis ground-based Etc. Courtesy of Häkkinen

4 SuperMAG network Courtesy for J. Gjerloev Measurements from over 200 observatories

5 Critical data gaps A need for few more magnetometer observatories? Yes, new observatories mainly needed to the southern oval region. Critical gaps also within northern hemisphere networks.

6 Tool for substorm research Algorithmical method for substorm identification

7 Recipe for substorm identification Step 1: IL index formed as a chain curve from all MIRACLE/IMAGE magnetometer measurements (in a similar way as AL is formed). Step 2: Negative bays identified algorithmically. Step 3: A set of substorm candidates visually checked. Step 4: The substorm list was tested against the manually created substorm list for 1997 and 1999 (Tanskanen et al. 2002).

8 Step 1: IL index formation The quietest period during each day was selected to determine the baseline. IL index is constructed by computing an envelope of the north-south (x) magnetic field components of the IMAGE measurements (Kallio et al. 2000). baseline

9 Different type of substorms

10 Challenges for the algorithmic search 1. Where does the substorm begin?

11 2. Where does the substorm end?

12 3. Where is the main substorms onset?

13 Step 2: Substorm identification A mechanistic method of substorm identification was developed to enable a study of large number of substorms (Tanskanen et al. 2005): (1) main substorm onset was sought by searching for a rapid decrease, more than 80nT in 15 minutes, leading to negative bay development. (2) The substorm was defined to begin when the first signs of the negative bay development showed up (at most 30 minutes before the onset). (3) The event was considered to end when IL index recovered to two tenth of the peak amplitude. The substorm candidates were rejected if they were shorter than 15 minutes, occurred outside the IMAGE range UT, or if the peak amplitude was less than 100 nt. Substorm candidates were combined if there was less than 10 minutes between them.

14 Result of the algorithmical search In total 5526 substorm during 11 years, which gives about 500 each year. Substorm occurrence rate varies from 433 (2001) to 652 (2003), being the largest about 3-4 years after the sunspot maximum. Sunspot maximum

15 Superimposed epoch curve A typical substorm created by computing a superimposed epoch curve. A subset of subtorms with peak amplitudes more than 200 nt showing a full recovery. Substorm onset used as a zero epoch time.

16 (Substorm)) research with magnetometers

17 Time scales Pi 2 pulsations Substorm expansion onset GIC Substorms High-speed streams Storms 27-day periodicity Annual and semiannual variation 11 and 22-year periodicity

18 Substorms during solar cycles 22 and 23 Substorms can grow up to several thousand nt (Halloween substorms). Average substorm peak amplitude for this data set was 400 nt (same average for manually and algorithmically searched substorm sets). maximum Sunspot minimum

19 Substorm activity parameter, Rsu Tanskanen et al., 2005 Substorm activity parameter characterizes the level of substorm activity in the Earth s magnetosphere. Substorm number is defined to be an average of substorm peak amplitudes of westward electrojet index. In this figure monthly averaging is used. Substorm number peaks in 1994 and 2003, which is about 3-4 years after the sunspot maximum.

20 Magnetic clouds Courtesy of Lepping et al Occurrence rate Average velocity

21 Interplanetary high-speed streams --> Substorm are driven by interplanetary high-speed streams.

22 Poleward vs. equatorward events 1 January, November, 1999

23 Latitudinal bins Substorms were categorized in the latitude bins according to the station where the maximum deviation of the X component was recorded Latitudinal zones from north to south (geogr. coord.) - north of south of 65

24 Stormtime vs.. non-storm substorms

25 Storm-time and non-storm substorms Tanskanen et al., ASR 2002; Viljanen,, Tanskanen and Pulkkinen,, AG, 2005 Non-storm Storm-time Site of maximum dh/dt i.e. substorm onset location is dramatically norther for non-storm than storm-time substorms. Typical storm-time substorm is about twice as intense and carries about 2.5 times more energy into the ionosphere than a typical non-storm substorm.

26 Future ICESTAR related-projects Make the algorithmically created substorm list publicly available. Create a list of the southern hemisphere negative bays with a similar method and compare results with northern hemisphere substorm activity.