Presentation on theme: "ASEN 5335 Aerospace Environments -- Geomagnetism 1 Paleomagnetism Natural remnant magnetism (NRM) of some rocks (and archeological samples) is a measure."— Presentation transcript:
ASEN 5335 Aerospace Environments -- Geomagnetism 1 Paleomagnetism Natural remnant magnetism (NRM) of some rocks (and archeological samples) is a measure of the geomagnetic field at the time of their production. Most reliable -- thermo-remnant magnetization -- locked into sample by cooling after formation at high temperature (i.e., kilns, hearths, lava). Over the past 500 million years, the field has undergone reversals, the last one occurring about 1 million years ago. See following figures for some measurements of long-term change in the earth's magnetic field.
ASEN 5335 Aerospace Environments -- Geomagnetism 2 Equatorial field intensity in recent millenia, as deduced from measurements on archeological samples and recent observatory data. ~10 nT/year
ASEN 5335 Aerospace Environments -- Geomagnetism 3 Change in equatorial field strength over the past 150 years
ASEN 5335 Aerospace Environments -- Geomagnetism 4 External Current Systems Sq consists of 2 parts: due to the dynamo action of tidal winds; and due to current exhange between the high-latitude ionosphere and the magnetosphere along field lines (see following figure). The solar quiet daily variation (Sq) results principally from currents flowing in the electrically-conducting E-layer of the ionosphere. Currents flowing in the ionosphere and magnetosphere also induce magnetic field variations on the ground. These field variations generally fall into the categories of "quiet" and "disturbed". We will discuss the quiet field variations first.
ASEN 5335 Aerospace Environments -- Geomagnetism 5 dusk dawn
ASEN 5335 Aerospace Environments -- Geomagnetism 6 Geographic Distribution of Magnetic Observatories
ASEN 5335 Aerospace Environments -- Geomagnetism 7 A Magnetogram
ASEN 5335 Aerospace Environments -- Geomagnetism 8 Local time northward eastward vertical Sq ground magnetic perturbations
ASEN 5335 Aerospace Environments -- Geomagnetism 9 Note: According to Ampere's law, a current will induce a magnetic field, and conversely a time-changing magnetic field will induce a current to flow in a conductor. Currents flowing in the ionosphere induce a magnetic field variation at the ground.... this is the "external" source we referred to before. But, some of this changing magnetic flux links the conducting earth, causing currents to flow there. These, in turn, induce a changing magnetic field on the ground which is also measured by ground magnetometers. These induced earth currents contribute about 25-30% of the total measured Sq field. The above mutual feedback is very much like "mutual inductance"
ASEN 5335 Aerospace Environments -- Geomagnetism 10 The Ionospheric Dynamo e-e- e-e- O+O+ B O 2 +, NO + VnVn equipotential line Global electrostatic field set up by dynamo action F-Region E-Region
ASEN 5335 Aerospace Environments -- Geomagnetism 12 Another quiet current system, the lunar daily variation, L, similarly exists because of lunar tidal winds in the ionospheric E-region. These are gravitational tides, as opposed to solar-driven (thermally-driven) atmospheric tidal oscillations. The L variation is about 10-15% of the Sq variation. L-variation M fgfg fcfc f g r -2 Centrifugal force experienced by Earth due to rotation about the Earth- Moon barycenter, located just inside the Earth
ASEN 5335 Aerospace Environments -- Geomagnetism 13 DISTURBANCE VARIATIONS storm-time variation, the average of X around a circle of constant latitude Disturbance local time inequality (“snapshot” of the X variation with longitude at a particular latitude) = storm time, time lapsed from SSC disturbed field X=X obs - X q =>D st ( ) + DS( ) longitude t=t’ disturbed value of a magnetic element (X, Y, H, etc.): In addition to Sq and L variations, the geomagnetic field often undergoes irregular or disturbance variations connected with solar disturbances. Severe magnetic disturbances are called magnetic storms. Storms often begin with a sudden storm commencement (SSC), after which a repeatable pattern of behavior ensues. However, many storms start gradually (no SSC), and sometimes an impulsive change (sudden impulse or SI) occurs, and no storm ensues.
ASEN 5335 Aerospace Environments -- Geomagnetism 14 Typical Magnetic Storm SSC followed by an "initial" or "positive" phase lasting a few hours. During this phase the geomagnetic field is compressed on the dayside by the solar wind, causing a magnetopause current to flow that is reflected in D st (H) > 0. During the main phase D st (H) < 0 and the field remains depressed for a day or two. The D st (H) < 0 is due to a "westward ring current" around the earth, reaching its maximum value about 24 hours after SSC. During recovery phase after ~24 hours, D st slowly returns to ~0 (time scale ~ 24 hours).
ASEN 5335 Aerospace Environments -- Geomagnetism 15 Various indices of activity have been defined to describe the degree of magnetic variability. For any station, the range (highest and lowest deviation from regular daily variation) of X, Y, Z, H, etc. is measured (after Sq and L are removed); the greatest of these is called the "amplitude" for a given station during a 3-hour period. The average of these values for 12 selected observatories is the a p index. The K p index is the quasi-logarithmic equivalent of the a p index. The conversion is as follows: The daily A p index, for a given day, is defined as
ASEN 5335 Aerospace Environments -- Geomagnetism 16 Long-term records of annual sunspot numbers (yellow) show clearly the ~11 year solar activity cycle The planetary magnetic activity index Ap (red) shows the occurrence of days with Ap ≥ 40 Ap and Solar Cycle Variation
ASEN 5335 Aerospace Environments -- Geomagnetism 17 AE is the "envelope" of deviations (of H from its quiet value) from a selection of high-latitude stations -- it is the difference between the curves AU and AL ("upper" and "lower") drawn through the max and min excursions of the deviations. Auroral Electrojet Index (AE)
ASEN 5335 Aerospace Environments -- Geomagnetism 18 Density Variation from Orbital Drag of a High-Latitude Low-Perigee (~150 km) Satellite vs. AE and Kp
When flowing through transformers, geomagnetically-induced currents my significantly increase the total transformer current leading to: Misoperation of relays Tripouts of individual lines Voltage drops Permanent damage to transformers Blackout of the whole system
Through tiny holes in the pipeline coating, pipeline steel may come into contact with the soil, water or most air and corrode. Corrosion Protection of Pipelines The corrosive electrochemical reaction can be inhibited by maintaining the pipeline steel negative (cathode) with respect to the surrounding soil (anode). In this arrangement the pipeline is the cathode of the circuit, and that is why it is called cathodic protection. The protection system keeps the pipeline potential with respect to the soil in a safe region (i.e., V) This is done by connecting the negative output of a DC power supply to the pipeline, and the positive output to the anode devices in the soil. This causes the electric currents to flow from the anode to the pipeline.
ASEN 5335 Aerospace Environments -- Geomagnetism 23 How Geomagnetic Variations Affect Pipelines Time-varying magnetic fields induce time-varying electric currents in conductors. Variations of the Earth's magnetic field induce electric currents in long conducting pipelines and surrounding soil. These time varying currents, named "telluric currents" in the pipeline industry, create voltage swings in the pipeline- cathodic protection rectifier system and make it difficult to maintain pipe-to-soil potential in the safe region. During magnetic storms, these variations can be large enough to keep a pipeline in the unprotected region for some time, which can reduce the lifetime of the pipeline. See example for the 6-7 April 2000 geomagnetic storm on the following page.
ASEN 5335 Aerospace Environments -- Geomagnetism 24 geomagnetic field variations at Ottawa magnetic observatory pipe-to-soil potential difference on a pipeline in Canada During the magnetic storm the pipe-to-soil potential difference went outside the safe region. That can increase the possibility of corrosion.