B field space

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Observational Space Physics The main goal of this course is to support the theoretical concepts of basic space physics by the corresponding observations in SPACE Going to space is still rather expensive, so we will NOT follow this teaching method

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Observational Space Physics 13.03-03.05.2007, given by Dr. Natalia Ganushkina (FMI) 5 credits 2 lectures per week: Tuesdays 12-14, Thursdays 10-12 Physicum D114 Exam in the end: verbal Lecturer is happy to answer any questions on Tuesdays 14-15, and Thursdays 12-13 Mobile: 050-341-2371, E-mail: Natalia.Ganushkina@fmi.fi You are also welcome to come to FMI!

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Observational Space Physics Outline of the course 1. Introduction: State of Space Physics before the spaceflight era and the beginning of satellite observations 2. Measurement techniques in space: fields, particles, waves, imaging 3. Regions and phenomena: Observations 4. Space weather 5. Current and future space missions

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Observational Space Physics is an easy course!

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Observational Space Physics Measurement techniques in space: Magnetic fields 1. Instruments for measurements of magnetic field in space 1.1 Characteristics of space magnetometers 1.2 What do we need to make the precise vector magnetic measurements in space? 1.3 Vector sensors: Fluxgate magnetometers 1.4 Search coil magnetometers and Optical Absorption magnetometers 1.5 Accurate measurement of zero levels or offsets 2. Modeling Earth's magnetosphere using spacecraft magnetometer data 2.1 Earth's magnetospheric magnetic field 2.2 Why do we need the Earth's magnetic field models? 2.3 The data-based approach 2.4 What models can 2.5 Examples of Tsyganenko models 3. Development of a magnetospheric magnetic field model (event-oriented) 3.1 How to construct event-oriented model? 3.2 Magnetic field data for modeling of storm event on November 6-7, 1997 3.3 Model results 4. Magnetic field measurements on the ground

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Instruments for measurements of magnetic field in space Instruments to measure the magnetic field in space use well-proven technique. For us nothing to worry about, just use the data!! Characteristics of space magnetometers

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What do we need to make the precise vector magnetic measurements in space? Magnetometer system with - linearity, - low noise, - stability and - accuracy. Spacecraft should be: - magnetically clean with a boom of suitable length for sensor mounting, - provide time and attitude telemetry to support high time resoluton magnetic field measurement data reduction. Elements of magnetometer system: - vector sensor and its associated circuits, - analog-to-digital convertor (ADC), - high fidelity data system The sensor should be: - highly linear, - with scale factors stable with temperature, - truly make vector measurements, - low noise level, - stable zero levela or offsets. The electronics operating the sensor should be: - simple and reliable, - with no limited life components. The data system should be: - have proper anti-aliasing filters for the data sampling rates used, - speed to process the sampled data quickly with a variety of algorithms. The ADC should be: - monotonic, - linear, - with high resolution.

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Vector sensors: Fluxgate magnetometers The majority of vector measurements in space have been made with fluxgate sensors. First fluxgate sensor in 1930th (Aschenbrenner and Goubau, 1936). Where they were used? - installed in aircraft to search for submarines, - to aid in making navigation charts, - for geophysical exploration. The 3-axis ring-core fluxgate sensor with a penny for comparison.

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Fluxgate magnetometers: How do they work? (1) The fluxgate magnetometer is based on what is referred to as the magnetic saturation circuit. Magnetic saturation refers to the induced magnetic field produced in the bars. magnitude of the inducing field H increases I=kH, k is the magnetic susceptibility magnitude of the induced field I increases Saturation occurs when increases in the strength of the inducing field no longer produce larger induced fields.

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Fluxgate magnetometers: How do they work? 2 parallel ferromagnetic bars wounded with a primary coil, with reversed direction, closeby. An alternating current (AC) through the primary coils causing varying magnetic field in each coil. Induced magnetic fields in the two cores, the same strengths but opposite orientations. Cores in an external magnetic field: - the magnetic field in one core parallel to the external field and so reinforced by it. - the other in opposition and so smaller. - the field will reach saturation in one core at a time different from the other core. Secondary coil surrounds the two ferromagnetic cores and the primary coil. Magnetic fields induced in the cores by the primary coil produce a voltage potential in the secondary coil. Absence of an external field: volltage = 0 Presence of an external field component: the behaviour in the two cores differs, by an amount which depends on the external field.

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Fluxgate magnetometers: They are good! Thus, 1. the fluxgate magnetometer is capable of measuring the strength of any component of the Earth's magnetic field by simply reorienting the instrument so that the cores are parallel to the desired component. 2. Fluxgate magnetometers are capable of measuring the strength of the magnetic field to about 0.5 to 1.0 nT. 3. These are relatively simple instruments to construct, hence they are relatively inexpensive ($5,000 - $10,000). Even this unclear scheme can be explained ‘by fingers’! Remember the previous slide?!

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Search coil magnetometers The simplest magnetometer, consists of an induction coil with a high-permeability core. Coil spins with the spacecraft and its axis is inclined with respect to the spin axis. The induced voltage is , where n is the number of windings of the induction coil and S it its effective cross section. The sensitivity of the search coil is the direct function of the number of turns of wire. Is used to measure fast magnetic field variations for electromagnetic waves. Uses the Zeeman effect in combination with optical pumping. Zeeman effect: splitting of atomic energy levels into sublevels in a magnetic field. The energy differences between the sublevels are proportional to the magnetic field strength. Optical pumping: the irradiating the gas with circularily polarized light whose frequency is chosen such that a transition to the excited state only occurs from the ground state sublevel. Optical Absorbtion magnetometer

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Accurate measurement of zero levels or offsets Accurate measurement of zero levels (offsets) by vector magnetometers has long been a problem. Offsets: - Offsets of sensors and sensor electronics with variations with time and temperature; - Fields of spacecraft (static or variable with time) add to offsets as measured by magnetometer. Methods: 1. On spinning spacecraft the sensors mounted normal to the spacecraft spin axis are easy to evaluate by averaging the data over several spin periods. 2. Electronic flipping: switches reverse the polarity of the sensor in such a manner that the offsets can be measured. 3. It was proposed using two triaxial magnetometers on one boom to determine the spacecraft field by measuring its gradient. Second magnetometer closer to the spacecraft is useful in identifying and calibrating spacecraft dynamic magnetic fields and provides redundancy. Today nearly all spacecraft of any size carry two magnetometers. Often the outboard magnetometer is for low field measurements and the inboard magnetometer for higher fields. The main thing which you need to know is that offsets exist and it is important to know them before using magnetic field data! How to know? Always ask the person responsible for particular data, PI (principal investigator) of instrument

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State of space physics before the spaceflight era: Early work on Geomagnetism 1839: Carl Friedrich Gauss: method for mathematically description of the Earth’s magnetic field by scalar potential expanded at any point (r, , ) in spherical harmonics: internal sources external sources Gauss and Wilhelm Weber founded a network of observatories expanded by British and Russians - obtained coefficients for internal sources - 99% of field originated inside the Earth But something did come from outside the Earth! 10 Deutsche Mark with Gauss portrait Well, history already as Gauss himself From Introduction

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Earth's magnetospheric magnetic field The magnetic field near the Earth: 97 - 99 % Main Field (From electric currents in the Outer Core) 1 - 2 % Crustal Field (From magnetized rock in the Crust) 1 - 2 % External Field (From ionized particles above the Earth) The Main Field: - near dipolar; - varies in strength from approximately 30,000 nT near the equator to 60,000 nT at the poles. - secular variation is about 1% per year; - north and south pole undergo a reversal (change in direction) every 500,000 years, on average. The External Field: - varies on time scales of seconds to days; - primarily due to solar interactions; - results from current systems; - range in intensity from fractions of a nT to thousands of nT.

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Magnetospheric magnetic field lines

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At which distances internal and external fields are most important? Internal field: up to about 6 Re External field: from about 6 Re everywhere

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Modeling Earth's magnetosphere using spacecraft magnetometer data (1) Why do we need the Earth's magnetic field models ? Modeling of the global geomagnetic field has a unique place in the Sun-Earth connection studies, since that field underlies all processes in the near-Earth space environment: - it links the interplanetary medium with the upper atmosphere and ionosphere, - guides energetic charged particles, - channels low-frequency electromagnetic waves, - confines the radiation belts and controls the auroral plasma, - directs electric currents, stores huge amounts of energy, intermittently dissipated in the course of magnetospheric substorms. Magnetic fields determine key properties of the geospace plasma, in particular, its anisotropy, which makes it possible to compare observations made in different regions of space by mapping them along the magnetic field lines. For this reason, understanding the properties of the geospace plasma requires knowing the structure of the geomagnetic field and its dynamics and relation to the state of the solar wind.

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Modeling Earth's magnetosphere using spacecraft magnetometer data (2) The data-based approach is to - Represent mathematically the field from each major current system, then add up the individual contributions. - Represent the expected response of the field to factors which can be determined, e.g., the orientation of the Earth's dipole axis, solar wind pressure, interplanetary magnetic field, and appropriate geophysical indices. - Calibrate the model against an extensive database of averaged magnetic field observations, tagged by simultaneous values of the solar wind parameters. We also use other data-based inputs, e.g. models of the average magnetopause based on observed boundary crossings. Data-based models serve as a bridge between theory and observations, and are the main guide to magnetospheric patterns; deservedly, their role has been compared to that of maps in the exploration of a new country.

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Modeling Earth's magnetosphere using spacecraft magnetometer data (3) What models can do Help experimenters - estimate where the field line from a given spot in space reaches Earth, - predict the magnetic field B expected at a given time and location in space. The predicted values may then be compared to actual observations or MHD simulations for similar conditions. - Help extract from data the greatest amount of information about the global magnetosphere. Help theorists - Understand the role of individual current systems in the dynamics of the magnetospheric field configurations. - Trace particle orbits and plasma convection flow, gaining an insight into the physical mechanisms of the magnetospheric dynamics. - Model the electric field/convection pattern in the polar ionosphere by mapping the solar-wind-induced potential along open field lines.

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Examples of Tsyganenko models for the Earth's magnetospheric magnetic field (1) Role of orientation of the Earth's magnetic axis with respect to the Sun-Earth line

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Examples of Tsyganenko models for the Earth's magnetospheric magnetic field (2) Role of the state of the solar wind, in particular, the orientation and strength of the interplanetary magnetic field , "carried" to the Earth's orbit from Sun

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Examples of Tsyganenko models for the Earth's magnetospheric magnetic field (3) Dynamical changes of the global magnetic field in the course of a disturbance: a temporary compression of the magnetosphere by enhanced flow of the solar wind is followed by a tailward stretching of the field lines.

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Event-oriented magnetospheric magnetic field modeling Now the real use of magnetic field data starts! We will develop a model for magnetospheric magnetic field for an event, called event-oriented. This is a real modeling, the method was published in Ganushkina et al., Journal Geophysical Research, 2002; Ganushkina et al., Annales Geophysicae.

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Choice of existing magnetospheric magnetic field model to modify: Any model easy to modify, simplest solution: Tsyganenko T89, Kp =4 (for storm events)  Replacing of T89 ring current with asymmetric bean-shaped ring current  Varying the global intensity of T89 tail current  Addition of thin current sheet  Scaling of magnetopause currents Determining free parameters for each current system Collecting input data:  All available magnetic field measurements during the modelled event in magnetosphere SYM-H measurements on the ground Varying free parameters, we find the set of parameters that gives the best fit between model and all available in-situ field observations How to construct event-oriented model?

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We model storm event on November 6-7, 1997 How many satellites do we have in the inner magnetosphere, where and when? First, we go to http://pwg.gsfc.nasa.gov/cgi-bin/gif_walk and plot 4 Day Combined Orbit, 50 Re As we can see, we have Interbal Tail and Geotail Also we know that there are GOES satellites and Polar satellites

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How to get magnetic field data from several satellites? go to Coordinated Data Analysis Web (CDAWeb) http://rumba.gsfc.nasa.gov/cdaweb/istp_public/ Select one or more sources: Geotail, Interball, Polar, Geosynchronous Investigations>GOES SUBMIT Select one or more instrument types: Magnetic fields (space)

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CDAWeb Data Selector CDAWeb Data Selector: GE_K0_MGF: Geotail Magnetic Field Instrument – S. Kokubun (STELAB Nagoya Univ., Japan) G8_K0_MAG: GOES 8 Magnetometer Key Parameters – H. Singer (NOAA SEC) G9_K0_MAG: GOES 9 Magnetometer Key Parameters – H. Singer (NOAA SEC) IT_K0_MFI: Interball Tail probe Magnetic Field, Key Parameters – S.Romanov (Space Research Inst., Moscow, Russia. ) PO_K0_MFE: Polar Magnetic Field,Key Parameters – C.T. Russell (UCLA) SUBMIT

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SUBMIT CDAWeb Data Explorer Select start and stop times from which to GET or PLOT data Select an activity: list or plot data GOES 8 Magnetometer Key Parameters – H. Singer (NOAA SEC) Available dates: 1995/12/01 00:00:30 – 2003/04/08 23:59:30 (Continuous coverage not guaranteed – heck the inventory graph for coverage) Magnetic Field (GSM) GOES-8 position, GSM Polar Magnetic Field,Key Parameters – C.T. Russell (UCLA) Available dates: 1996/03/16 00:00:55 – 2006/04/30 23:59:48 (Continuous coverage not guaranteed – check the inventory graph for coverage) Observed vector magnetic field in cartesian GSM (0.92 min res.) Time axis label: POLAR position, 3 comp. in cartesian GSM (0.92 min res.) 2 examples

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Output: Plot data

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1. Time 2. Magnetic Field (GSM) 3. GOES-8 position, GSM EPOCH BX GSM BY GSM BZ GSM GSM X GSM Y GSM Z dd-mm-yyyy hh:mm:ss.ms nT nT nT km km km 06-11-1997 00:00:30.000 -9.73266 -53.2391 79.4975 -13236.1 39899.4 3193.35 06-11-1997 00:01:30.000 -9.44240 -53.2355 79.4753 -13403.4 39849.2 3123.54 06-11-1997 00:02:30.000 -9.21197 -53.2042 79.4829 -13570.5 39798.1 3053.54 06-11-1997 00:03:30.000 -9.12850 -53.1896 79.4141 -13734.6 39747.1 2984.06 06-11-1997 00:04:30.000 -8.94674 -53.0465 79.5183 -13901.2 39694.4 2913.73 06-11-1997 00:05:30.000 -8.64843 -52.9703 79.6556 -14067.6 39641.0 2843.23 06-11-1997 00:06:30.000 -8.36586 -52.8765 79.6646 -14233.5 39586.8 2772.62 Output: List data GOES 8

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Storm event on November 6-7, 1997: Model results

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Storm event on November 6-7, 1997: Model results

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Storm event on November 6-7, 1997: Model results

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Storm event on November 6-7, 1997: Model results

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Homework for magnetic field measurements in space Event: May 4, 1998, 00-24 UT 1. Find out which satellites were in the inner magnetosphere during that event and when. Criterion: satellite orbit inside Xgsm = [-20 Re, 10 Re] Ygsm = [-15 Re, 15 Re] Zgsm = [-15 Re, 15 Re] Output: Satellite name, UT interval when it was inside inner magnetosphere 2. Find out if magnetic field data were available during those UT intervals. Go to Coordinated Data Analysis Web (CDAWeb): http://rumba.gsfc.nasa.gov/cdaweb/istp_public/ Chose List data option Copy output data files and plot them as Bx_gsm(UT), By_gsm(UT), Bz_gsm(UT)