# Presentation: 162891

## Magnetoseismic Research through the Observations by Ground Magnetometer Networks

**Peter Chi**
- Institute of Geophysics and Planetary Physics
- UCLA
- IAGA Workshop on Magnetic Observatories
- Golden, Colorado, June 16, 2008

**Notes:**
Very grateful to have the opportunity
Magnetoseismology is the topic I have been indulging myself i, and I wish you find some part of the presentation interesting 
Thank a collaborator of mine Dong-Hun Lee for providing numerical simulation results

## Introduction

**Magnetoseismic methods are becoming popular in space weather research, and the major data source is ground magnetometers sampling data at ~1 sec (Waves of interest: Pc 3-5; Pi 2)**
**As in seismic research, magnetoseismic analysis requires data from a network of stations.**
**In this presentation I will discuss:**
**Principles of two magnetoseismic methods**
**Examples of applications**
**Future directions**

## Two (Magneto)-Seismic Methods

|  | Seismology | Helio-seismology | Magneto-seismology |
| --- | --- | --- | --- |
| Travel-time Method |  |  |  |
| Normal-mode Method |  |  |  |

**Notes:**
Our work at IGPP has confirmed that both methodologies can be used in magnetospheric physics.
(We also insert the definition of magnetoseismology in Wikipedia)
In appearance the major use of magnetoseimology is to infer magnetospheric density
We find that there are challenging physics questions to investigate, and that’s why we like to spend time on it.
(Add something useful to day ...)

## Waves

**Notes:**
Body waves:
P wave: compress and dilate
S wave: shears
MHD waves: (usually oriented as B pointing up)
Compression mode: energy propagates in the direction of k, density perturbations
Shear Alfven wave: energy propagates along B
Typical numbers of speeds
Body waves: several km/s
MHD waves: hundreds or thousands of km/s

## Normal-mode Magnetoseismology

**Notes:**
Analogy to plucking harp strings
Magnetic field lines as harp strings – frequency depends on length of field line and density
Additional feature (important) for continuum: phase change across resonance point

## Detecting FLR frequencies from the Ground

**Detecting FLR frequencies from the Ground**

**Requires small separation between adjacent stations:**
**80-400 km in the north-south direction  **

## Mid-continent Magnetoseismic Chain (McMAC)

- ● Field line resonance method requires close separation between ground stations in the north-south direction.
- ● The mean north-south separation between two adjacent McMAC stations is 275 Km.
- ● Joint operation with CANOPUS Churchill Line (Canada), IGPP-LANL (U.S.) and MAGDAS (Japan) provides the magnetic field data _**from **__***L***__** = **__**1.2**__** **__**to **__**11**__**+**__** at one local time.**_

**Mid-continent Magnetoseismic Chain (McMAC)**

**Notes:**
World map – greatest latitudinal coverage is in North America
A lot of thinking has been devoted to finding a serious name
Explain L-value
The distance to the Earth’s center when the magnetic field line crosses the equator
Can indicated anywhere along the field line, including the ground station location

## Density Variations during Magnetic Storms 

**Density Variations during Magnetic Storms **

- Depletion (*Chi et al.*, 2000)

- Enhancement (*Chi et al.*, 2005)

**Dst**

***f***

******m**

**TEC**

**FLR Freq (n=1) [mHz]**

**Dst**

**Log Equatorial Density [amu/cm****3****]**

** ****Significant influence from the ionosphere to the plasmasphere during magnetic storms**

**Notes:**
Suggesting the ionospheric influence to the plasmasphere during magnetic storms

## Equatorial Mass Density Derived from Ground Field Line Resonance Observations

**Equatorial Mass Density Derived from ****Ground Field Line Resonance Observations**

- October 28, 2003

**Use of Data:**
** ****Understand the sources of sinks of magnetospheric plasma**
** ****Magnetic storms**
** ****Interaction with the ionosphere**
** ****Investigate the contribution of heavy ions (e.g. He****+****, O****+****) by comparing with other charge density measurements**

- Y[RE]

- 0

- 2

- 4

- 6

- 8

- -2

- -4

- -6

- -8

**From ground FLR**

**From satellite**

**Notes:**
FLR results can cover dayside region very well
EUV images can 

## Normal-mode Analysis at Multiple Local Times

-  Decent observational coverage for magnetoseismic research in North America and Europe; Magnetometers in East Asia are separated too widely to conduct cross-phase (FLR) analysis.
-  Establishing a magnetoseismic chain in Siberia can enable nonstop FLR observations: At least one magnetoseismic chain is located in the dayside at all times.

## Travel-time Magnetoseismology

**Travel-time ****Magnetoseismology**

- Spacecraft
- Ground stations

**(Note the observations are made within the body of interest)**

**Notes:**
Imagine an impulse propagating in the magnetosphere
observed by satellite and ground stations at different locations
Two critical things to know:
Travel speed
Travel path
These two pieces of information lead to travel time

## Sudden Impulses

**Sudden Impulses**

- Energy from enhanced dynamic pressure of solar wind
- Excites impulses (or quakes) at the front side of magnetosphere

**Can’t use ray tracing in magnetoseismology !!!**

- Scale Comparison

|  | Earth & Seismic Waves | Magnetosphere & MHD Waves |
| --- | --- | --- |
| Radius | 6400 km | ~64,000 km (sunward) |
| Wave speed | 1-14
km/s | 300-10,000 km/s |
| Wave-length | ~100 km | 104-105 km |
| Scale length of V | Typically
 103 km | Typically
 2104 km |

**Notes:**
Energy from sudden impulses –
Solar wind dynamic pressure can change from 2 nPa to ~10 nPa
(Ref.: total particle energy in an aurora ~10^21 ergs = 10^14 Joules)
An 5.5 Richter magnitude earthquake has senergy of ~10^20 ergs (= 10^13 Joules)
6.9 M = 1.4 x 10^15 Joules; 9.2 M = 4 x 10^18 Joules
A 10 nPa solar wind pressure pushing dayside magnetopause (5 Re x 5 Re effective area) for 5 Re: ~ 6 x 10^13 J (perhaps a magnitude 5 earthquake)

## Tamao’s Model of Impulse Propagation via MHD Waves

- Tamao [1964]

**Compressional Mode:**
** ****Propagates in all directions**
** ****Suffers geometrical attenuation (~ 1/r)**
**Alfven Mode:**
** ****Propagates along magnetic field line**
** ****Wave energy conserved**

- Compressional

- Alfven

**“****Tamao travel path” : The path that preserves most wave energy**

**Tsutomu**
**Tamao**

**Notes:**
Main ideas:

## Travel-Time Magnetoseismology

- Parameters to solve: (*t*0,) *n**mp*, *m*1, *L**pp*, *n**pp*, *m*2

- Density Function

- Inversion

- (*L**pp*: plamapause *L*-value)

- (*t*0: start time)

- Tamao Model of Wave Propagation

*a*

*b*

*c*

*d*

*e*

- Alfvén
- Velocity

- Magnetic Field: Tsyganenko Model

**Notes:**
Tamao travel paths
Tamao travel time
Wave velocity (Alfven speed) is a function of density and magnetic field
Inversion: estimating the parameters by comparing modeled travel time and observed arrival time
When the source location is known (by examining the solar wind data) the parameters to solve are only associated with density function

## Pi 2 Impulses from the Nightside

** ****Ground magnetometers stations in Canada and the U.S.**
** ****Approximately along 265 degrees longitude (all events within 3 hours from midnight)**

- [Kepko et al., 2001]

## Event Timeline

- t

****

- Pi 2 starts in the tail

- Auroral breakup

- Conventional Pi2
- onset time (ground)

- Start time of putative
- current disruption

-                 0
- (=06:53:30 UT)

- :30

- 1:00

- 1:30

- 2:00

- 2:30

- mm:ss

## Summary

**1-sec systems enable Ultra-Low-Frequency research, including magnetoseismology**
**Magnetoseimic analysis has become a major driving force of new space-weather-oriented magnetometer arrays.**
**Normal-mode analysis:**
**Requires observations from two stations separated by ~100 km in the north-south direction **
**Observational coverage in **_**East Asia**_** needs improvements.**
**Travel-time analysis:**
**Prefers a wide spatial observational coverage – usually needs coordinated observations across multiple arrays**
**Both magnetoseismic methods require sufficient time accuracy of magnetometer measurements:**
**A sufficient requirement: 0.1 sec**

## Impulsive Events from the Nightside

- [Kepko et al., 2001]

**Pi 2**
**(Irregular pulsations, Type 2; *****T***** = 45-150 sec)**

**Substorms: Explosions**
**in the magnetotail**

**Notes:**
Frequency of sudden impulses is only once per weeks
SI is useful for understanding dayside density
Pi 2 occurs more often (once per a couple of hours)
Pi 2: Irregular pulsations Class 2
Pi 2 is related to the onset of substorms
Pi 2 is also found related to bursty bulk flows

## Model Tamao Travel Time

- Fast mode
- Alfvén

**Tamao**
**travel path**

**Travel Time**

**Source at X = -25 Re**

**Source at X = -15 Re**

**Source at X = -20 Re**

**Source at X = -10 Re**

## Inversion Results

| Pi 2 | Ls | Nsh[cm-3] | Tsh [keV] | t0 |
| --- | --- | --- | --- | --- |
| Event 1 | 17.1 | 1.04 | 3.84 | 04:03:40 |
| Event 2 | 18.7 | 0.23 | 1.73 | 05:13:20 |
| Event 3
(substorm onset) | 22.3 | 0.23 | 3.85 | 06:53:40 |
| Event 4 | 20.2 | 0.30 | 2.99 | 07:23:20 |

- Source distance

- Start time

-  All observations are made by ground magnetometers
-  Only stations with L  15 are used in inversion; One station at L = 29 is not used.