The Activation Energies of Temperate Snow Samples

Abstract

ABSTRACT. The temperature depe ndence of the high-frequency conduc tivity o f snow was studied for eight samples in the range -lOO C to -80°C. The activation e n ergies for granula r snow varied from 0.60 eV a t tempera tures d o wn to -25°C to 0.24 eV at temperatures below -25°C, a nd fo r dry snow fro m 0 .42 eV to 0. 18 eV in the sam e tempera ture r a nges. Dry snow samples had higher co nductiv ities a nd lower activa tion energies tha n gra nul a r icy samples throughout the comple te tempera ture range.Volume and surfacc conducti on processes were operative. At the higher te mperatures above -25°C volume condu ction was a ttributed to Bj errum defect migrati on, while at lower temperatures Bj erTum conduction was re placed by ionic-defect conduction. Granu la r samples which h ad experienced m e lting and refreezing on th e surface of the crystals have a structural c h ange which d ecr eased the ion ic-defect density a t the surface a nd lowered the sample co ndu ctivity.Above -25°C, the ac tiva tion energies for granula r icy snow were consis te nt with those for temperate g lacier ice and fo r laboratory singl e a nd pol ycrys ta llin e ice sampl es, but was not in agreement with those of either pol ar snow o r ice. Below -25°C, the activa tion ene rgies for gra nular sn ow we re consistent with those for temperate glacier ice, containing impurities, and for pola r snow a nd ice. F resh dry tempe ra te snow sa mples had lower activation energi es than ill situ polar snow. RESUME . Energies d 'activation des echantillolls de lIeige temp'rie.La depend e nce avec la tem perature d e la conductivite ha ute frequen ce de la neige a e te etudiee avec huit echa ntillons d a ns le d om aine d e temperature compTis entre -10 et -80°C. Les e ne rgi es d' acti va tion pour la neige gran ul aire vari ent de 0,60 eV pour des temperatures superieures it -25°C it 0,24 eV pour des te mpera tures infe ri e ures et, dans le cas de la ne ige seche, d e 0,42 eV it 0, 18 eV dans les m emes doma ines d e tempera ture. L es ech a ntillons d e neige seche presentent une conduc tivite plus elevee et un e energi e d 'ac tiva tion plus fa ibl e que les echantillons d e neige g r an ulaire it toutes te mpera tures.D es processus d e conduction en volume e t en surface inte r vie nnent. Au-d ess us de -25°C, la co nduction e n volume est attribuee it la migration d es defa uts de Bj e rrum tandis qu'aux temperatures plus basses predomin e la conduc ti o n par defa uts ioniqu es. L es echa ntillons gra nula ires qu i o nt subi un e fusion e t un regcl e n surface, presentcnt un changement s tru c tura l correspondant it un e decroissa n ce d e la densite d e defauts ioniques it la surface e t, ains i, une dimin u tio n de la conduc tivite.A u-dessus d e -25°C, les energi es d'ac ti vation d e la ne ige g ra nul a ire sont e n accord avec cell es obsen 'ees dan s le cas d e la g lace d e gl aciers temperes o u d a ns le cas d 'echantill ons mono-o u pol ycristallins de g lace d e la bora toire; cll es so nt, par contre, diffe re ntes...

A Critical Review of the Low-Frequency Electrical Properties of Ice Sheets and Glaciers

“…Much higher activation energies are measured in temperate ice (e.g., Reynolds and Paren, 1984) and firn generated from laboratory-grown ice (e.g., Paren and Glen, 1978). The activation energies of snow and hoar frost collected outside the world's cold regions are apparently strongly dependent on snow type, frequency, and temperature, having values between ,0.2 eV and .1 eV (e.g., Traub and Gribbon, 1978;Takei and Maeno, 2001). Much of the surface of the world's ice masses is either permanently or at least during some parts of the year covered by firn or snow of lower density.…”