MEA­SUR­ING THE AURORA

Cre­at­ing a home­made mag­ne­tome­ter al­lowed Stu­art Green to keep a record of space weather, de­spite cloud and his southerly lo­ca­tion

Sky at Night Magazine - - CONTENTS - ABOUT THE WRITER A com­pos­ite ma­te­ri­als engi­neer, Dr Stu­art Green is a keen so­lar as­tronomer and space weather en­thu­si­ast

We can’t see the aurora in much of the UK, but we can hear it. Find out how…

E arth is bathed in a con­stant stream of en­er­getic par­ti­cles orig­i­nat­ing at the Sun. This so­lar wind ebbs and flows, and oc­ca­sion­ally explodes in a coro­nal mass ejec­tion, throw­ing bil­lions of tonnes of our star’s plasma into space, which im­pacts on Earth’s mag­ne­to­sphere – our pro­tec­tive mag­netic bub­ble. The re­sult is bright and beau­ti­ful au­ro­rae, the lu­mi­nous splen­dour of which is our only vis­ual con­fir­ma­tion of this Earth-Sun con­nec­tion. For those for­tu­nate enough to wit­ness this spec­ta­cle, the last­ing im­pres­sion is one of awe at its mag­nif­i­cence. For the rest of us, such events are lost save for the im­ages avail­able on­line, through which we can only ex­pe­ri­ence the au­ro­rae vi­car­i­ously.

A few years ago I was look­ing for a way to es­tab­lish a de­gree of con­nec­tiv­ity with the Sun that was oth­er­wise un­avail­able to me in my lower lat­i­tude lo­ca­tion in the UK. That’s when I thought about build­ing a mag­ne­tome­ter. Not only does the so­lar wind cre­ate the aurora, but in the process of be­ing de­flected by our pro­tec­tive mag­ne­to­sphere, it also im­prints its sig­na­ture upon our pro­tec­tive bub­ble. This is de­tectable at any point on our planet with a mag­ne­tome­ter, which picks up the mag­netic sig­na­ture as it fluc­tu­ates ac­cord­ing to the strength, speed and mag­netic ori­en­ta­tion of the pass­ing plasma.

Mag­ne­tome­ters can be pur­chased com­mer­cially, of course, but my in­ter­est lay in build­ing one for the fun of it and for the chal­lenge. The ba­sic scheme of the de­sign is a highly sen­si­tive mag­netic sen­sor, an ul­tra­sonic emit­ter, an ul­tra­sonic-toau­dio fre­quency con­verter and a com­puter with a sound card and Spec­trum Lab au­dio spec­trum anal­y­sis soft­ware for data log­ging.

A sta­ble en­vi­ron­ment

At the heart of this setup is the mag­netic sen­sor. Called a flux­gate, th­ese sen­sors can be ex­tremely sen­si­tive and are per­fectly suited to mea­sur­ing the tiny per­tur­ba­tions in the lo­cal mag­netic field caused by space weather. The par­tic­u­lar sen­sor I used pro­vides an out­put fre­quency that varies ac­cord­ing to the strength of the mag­netic field.

As well as re­spond­ing to the mag­netic field, the out­put fre­quency also changes with tem­per­a­ture, so the sen­sor has to be lo­cated in a tem­per­a­turestable en­vi­ron­ment. My sen­sor is buried in my gar­den about 0.5m be­low the sur­face. It is also pro­tected by a wa­ter­proof hous­ing made from 40mm di­am­e­ter plumb­ing pipe fit­ted with end caps sealed us­ing suit­able sol­vent.

The sen­sor is po­si­tioned al­most per­fectly level on a poured con­crete base and points along Earth’s mag­netic east-west di­rec­tion. It’s also sited away from any stray mag­netic fields that might be cre­ated by lo­cal elec­tric ca­bles and equip­ment. The flux­gate sen­sor’s power sup­ply also has to be sta­ble to avoid spu­ri­ous read­ings, so mine is linked to a

lin­ear volt­age reg­u­la­tor that con­verts 9V DC from a mains transforme­r to a sta­ble 5V DC sup­ply.

When point­ing east-west, the out­put from the flux­gate sen­sor is a train of +5V square wave pulses at a fre­quency in the range of 60-70kHz, which is sig­nif­i­cantly above the op­er­at­ing fre­quency of any stan­dard com­puter sound card. So I turned to equip­ment in­tended for zo­ol­ogy and con­nected the out­put of the mag­netic sen­sor to a bat de­tec­tor with an ul­tra­sonic trans­ducer emit­ter.

Bat de­tec­tors are de­signed to con­vert ul­tra­sonic bat calls into au­dio fre­quen­cies that we can hear. Com­monly this is ac­com­plished by a process called het­ero­dyn­ing, in which an in­ter­nally gen­er­ated, tune­able ref­er­ence fre­quency is mixed with the vary­ing ul­tra­sonic in­put sig­nal from the bat to cre­ate au­di­ble sound at a fre­quency equal to the dif­fer­ence be­tween in­put and ref­er­ence fre­quen­cies.

Fre­quency range

For the pur­poses of mag­ne­tom­e­try, this fre­quency dif­fer­ence should be tuned to a sin­gle au­di­ble tone that can be recorded with a stan­dard com­puter sound card. I found 3-4kHz to be a suit­able range. The other ad­van­tage of us­ing a bat de­tec­tor is that the au­dio out­put is of good fidelity, mean­ing that sub­se­quent data anal­y­sis can be ac­com­plished at high res­o­lu­tion. For this project, I used a de­tec­tor

with an in­ter­nal crys­tal os­cil­la­tor for pre­ci­sion and to min­imise any fre­quency drift. This de­tec­tor is usu­ally pow­ered by a set of bat­ter­ies, but as that only pro­vided a bat­tery life of less than 24hrs, the bat­ter­ies were swapped for another sta­bilised DC feed from the 9V mains transforme­r.

Gath­er­ing data

With the de­tec­tor set up I was ready to log data, and for this I used free soft­ware called Spec­trum Lab, writ­ten by Wolf­gang Buescher (www.qsl. net/dl4yhf/spec­tra1.html). This en­ables the data stream to be dis­played as a con­tin­u­ous chart as the day pro­gresses. The soft­ware can also send mea­sured fre­quency data to a com­puter hard drive at pre­s­e­lected in­ter­vals, to­gether with a time and date stamp, build­ing up a data­base through­out an en­tire ge­o­mag­netic event. Later, this data­base can be ex­ported as a CSV file and copied into an Ex­cel spread­sheet for fur­ther anal­y­sis. Any time pe­riod be­tween logged events can be se­lected; I use a onesec­ond ca­dence for high res­o­lu­tion work and a 150-se­cond ca­dence for stan­dard res­o­lu­tion work.

Analysing the data in­volves con­vert­ing fre­quency to mag­netic field strength (more ac­cu­rately mag­netic flux den­sity in nano-Tesla) us­ing con­ver­sion fac­tors pro­vided by the sen­sor man­u­fac­turer, and then chart­ing the re­sult as a func­tion of time.

As it turned out, my mag­ne­tome­ter needed a few re­fine­ments to make it more con­sis­tent with pro­fes­sional data, in par­tic­u­lar bet­ter tem­per­a­ture con­trol of the bat de­tec­tor. The out­put from the de­tec­tor was be­ing sig­nif­i­cantly im­pacted by small am­bi­ent tem­per­a­ture fluc­tu­a­tions that were im­print­ing on the out­put fre­quency. It was only when the de­tec­tor was placed in a tem­per­a­ture­de­fined en­vi­ron­ment (in­side a vac­uum flask, in­side a cool box fit­ted with a vi­var­ium heater mat at a con­trolled tem­per­a­ture) that the out­put met the pro­fes­sional read­ings.

The out­put from such a rel­a­tively sim­ple de­vice is re­mark­ably con­sis­tent with data gen­er­ated by pro­fes­sional ge­o­mag­netic mon­i­tor­ing sta­tions such as Eskdale­muir, op­er­ated by the Bri­tish Ge­o­log­i­cal Sur­vey, and its sen­si­tiv­ity and res­o­lu­tion cer­tainly com­pete favourably for the hob­by­ist with the out­put from mag­ne­tome­ters cost­ing con­sid­er­ably more.

Hav­ing es­tab­lished my home­made mag­ne­tome­ter I now have that con­nec­tion with the cos­mos that I was look­ing for, with data stream­ing into my PC con­stantly, cap­tur­ing the ebb and flow of the so­lar wind and oc­ca­sional coro­nal mass ejec­tion that buf­fets our planet.

“I now have that con­nec­tion with the cos­mos that I was look­ing for, with data stream­ing into my PC con­stantly, cap­tur­ing the ebb and flow of the so­lar wind"

Elec­tri­cal pulses ~70kHz Mag­netic sen­sor Ul­tra­sonic emit­ter Sound waves ~70kHz Elec­tri­cal pulses ~3.5kHz Ul­tra­sonic to au­dio con­verter PC or lap­top via sound­card Spec­trum Lab Au­dio spec­trum anal­y­sis soft­ware The mag­ne­tome­ter is made up of a mag­netic...

Tem­per­a­ture sta­bil­ity is vi­tal, hence the mag­ne­tome­ter (left) is housed in a vac­cuum flask (cen­tre), which is kept in­side cool box (right)

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