E field

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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


Observational Space Physics 13.03-03.05.2007, given by Dr. Natalia Ganushkina (FMI) 5 credits 2 lectures per week: Tuesdays 14-16, Physicum D112, Thursdays 10-12, Physicum D114 Exam in the end: verbal Lecturer is happy to answer any questions on Tuesdays 13-14, and Thursdays 12-13 Mobile: 050-341-2371, E-mail: Natalia.Ganushkina@fmi.fi You are also welcome to come to FMI!


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: magnetic and electric fields, particles, waves, imaging 3. Regions and phenomena: Observations 4. Space weather 5. Current and future space missions


Observational Space Physics Measurement techniques in space: Electric fields 1. Instruments for measurements of electric field in space 1.1 Double probes for measurements of electric fields 1.1.1 Observations in dense plasma 1.1.2 Observations in tenuous plasma 1.2 Electron beam experiment 1.2.1 The beam experiment on GEOS satellite 1.2.2 Beam experiment on Cluster satellite 1.2.3 Observations of electric field component parallel to magnetic field 2. Electric field experiment (EFI) on Polar 2.1 Polar satellite, instruments 2.2 EFI objectives 2.3 EFI data example 3. Electron drift instrument (EDI) on Cluster 3.1 EDI objectives 3.2 High resolution survey plots 3.3 Scientific results 4.


Instruments for measurements of electric field in space It is difficult to perform accurate electric field measurements! The electric field is generally weak, important corrections have to be applied for measurements provided by instrument Characteristics of electric field instruments


Double probes for measurements of electric fields: Observations in dense plasma (1) The electric field probe measures the difference of the electric potentials at two locations in space. Consists of two electrodes usually of spherical shape, fixed on booms at a sufficiently long Distance (40 m) from spacecraft. Instrument provides electric field component perpendicular to spacecraft spin axis, determined from the potential difference U between two electrodes P1 and P2 and their separation length l. Up1 and Up2 are the plasma potentials at the location of electrodes. D is the diameter of the region of plasma potential decrease around P1. Uw


Double probes for measurements of electric fields: Observations in dense plasma (2) Potential alterations: 1. Electrodes alter the plasma potential so that potential decreases Uel because electron current je in the surrounding plasma towards the electrods is larger that the ion current jp. 2. Alteration of the measured potential Uw due to the energy W gained if an electron penetrates into the electrodes. Alterations vary with time and location. Uw


Double probes for measurements of electric fields: Observations in dense plasma (3) Measured potential difference U related to plasma potential difference Up1 – Up2 as: Correction terms are NOT small compared to the main terms! Methods to separate the main terms from the correction terms Rotating spacecraft Signal from plasma potential difference varies with the phase angle of rotation, The correction terms remain constant. Additional induced term due to spacecraft rotation can be determined and included into corrections.


The ion current from ambient plasma to electrodes is negligible in tenuous plasma. The electron current still exists. Largest impact from secondary electrons produced by: - photo emissions, - interaction with energetic particles impinging on electrodes. Source of errors due to secondary electron emissions: - electrode potential becomes positive relative to plasma potential, - small changes of current density lead to large changes of measured potential Reducing by: - application of a bias current to electrodes - probe potential to small variations, - a bias current also applied to the satellite to avoid asymmetric effects of photoelectrons. Double probes for measurements of electric fields: Observations in tenuous plasma


Electron beam experiment Early version of electron beam experiment: measurements of the electric field E perpendicular to the induction B of the ambient magnetic field. Motion of electron in homogeneous magnetic field: Circular orbit, gyro period , m is the electron mass and q is the charge. With additional electric field E: Drift speed is: electron drifts perpendicular to electric and magnetic fields. Orbit is not circular, there is a displacement . Example: For E=10-3 V/m, B=100 nT, =3.6 m In the electron beam experiment, displacement  is measured and electric field is then calculated


The beam experiment on GEOS satellite Electron source for the beam located on a boom at distance D from the satellite. Detector to monitor the returning beam located in the main body of satellite Electron beam emitted perpendicular to the boom. Angle  between E and perpendicular to the boom varies from 0 to 360 for each satellite rotation. The returning beam displaced along the boom by distance  If 0 > D, returning beam is observed at spacecraft during two positions when  = D. The direction of E is derived from the phase angle  corresponding to tg, between – 0 and + 0


Beam experiment on Cluster satellite Considerably improved version of the beam experiment on GEOS satellite. Improvements: - Electron beam is steerable in azimuth and elevation, - High-time resolution measurements are possible, - Instrument can be used for large number of magnetic field directions. Two antiparallel beams Electric field can be determined from the difference between flight times In addition, gyro period can be determined from the sum of times of flight and the magnetic field can be obtained:


Observations of electric field component parallel to magnetic field Electric field component parallel to magnetic field is very important for many magnetospheric disturbance processes. Indirect method to measure: - Electron beam is ejected from Spacelab, - Electrons spiral around magnetic field lines against the direction of electric field force, - They are finally mirrored by this force and return. Returning electrons can be observed - Directly on subsatellite - Indirectly by secondary effects Electric field is calculated from the observations of returning electrons Actual measurements: - Beam generates plasma instabilities, - Any simple deduction of electric field not possible!


Polar satellite


Electric Field Instrument (EFI) on Polar: Objectives Electric Field Instrument (EFI) measures along the Polar orbit: - vector electric field - thermal electron density. The electric fields may be related to: - motion of magnetic fields that are carried past the spacecraft with the ambient plasma (convection electric fields), - structures or waves associated with the aurora. They can be used to study: - the origin and nature of auroral zone processes such as energetic electron precipitation into the atmosphere; - generation of auroral electromagnetic noise emission; - Their role in the deposition of solar wind energy into the magnetosphere, ionosphere, and upper atmosphere through their ability to accelerate particles either directly (in the case of charged particles) or through collisions (between accelerated charged particles and ambient neutral particles); - manner in which the different scale structures (large-scale (hundreds to thousands of km) and imbedded small scale (<1 km) structures and waves are related


Electric Field Instrument (EFI) on Polar: Objectives The Electric Field Instrument (EFI) on the Polar spacecraft is designed to measure the vector electric field and thermal electron density at the spacecraft location. Provide measurements along the 2 Re X 9 Re orbit of vector electric fields in the amplitude range of 0.02 to 1000 mV/m, and cold electron densities in the range between 0.1 and 100 cm-3. Includes three sets of double spherical probes: two pairs extend to separations of 100 m or more on wire booms in the spacecraft spin plane, third pair is spaced along the spacecraft spin axis on rigid booms that keep them 14 m apart.


Description of the EFI Key Parameters. (1.) Spin Plane Electric Fields (ESPIN) This parameter represents the "raw" electric field in the spin plane as measured by the potential difference between opposing spheres 1 and 2. The units are mV/m. (2-4.) DC Electric Fields (EXY, EZ) The DC Electric Field is computed on the spacecraft from the data collected from the "1-2" boom pair and constitutes two electric field components in the spin plane, Exy(12)S and Ez(12)S. The units are mV/m. (5.) Spacecraft Potential (POTENT) (6-8.) Bandpass Filter (9.) Probe Bias Current etc Electric Field Instrument (EFI) data example


Electron Drift Instrument (EDI) on CLUSTER: Objectives EDI instrument is able to make accurate and highly sensitive measurements of the electric field and of the perpendicular gradient of the magnetic field. Cluster has been designed primarily to study small-scale structures in three dimensions in the Earth's plasma environment. The processes leading to the formation of such structures are believed to be fundamental to the key processes of interaction between the solar wind and the magnetospheric plasmas. With Cluster it is possible to obtain differential quantities by measurements of particle and field properties at the four spacecraft locations. These differences can be used to form quantities such as the gradient, curl, and divergence of the fields, and of the plasma moments such as velocity and pressure.


High resolution survey plots from EDI


Average electric field magnitudes (arrow lengths) and convection directions (arrow orientation) from approximately 15 months of Cluster EDI data. The measurements were mapped to the equatorial plane and binned using 1-hr local time intervals and 1 RE distance intervals from 4 to 10 RE. The four panels correspond to BZ<0 (top), BZ>0 (bottom), corotating frame (left) and inertial frame (right). Convection electric field at 4-10 Re

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